docs: add SessionBackend abstraction design specs

.spec/SessionBackends/00-overview.md

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+# Session Backend Abstraction — Design Overview
+
+## Spec Documents
+
+| Doc | Contents |
+|-----|----------|
+| [01-tmux-audit.md](01-tmux-audit.md) | Every tmux touchpoint in the codebase, categorized |
+| [02-session-backend-interface.md](02-session-backend-interface.md) | `SessionBackend` interface, data model changes, migration plan |
+| [03-tmux-backend.md](03-tmux-backend.md) | `TmuxSessionBackend` implementation, method mapping, file layout |
+| [04-ambient-backend.md](04-ambient-backend.md) | `AmbientSessionBackend` implementation, API mapping, behavioral differences |
+
+## Summary
+
+### What exists today
+
+- **tmux hardcoded everywhere**: 10+ functions in `tmux.go`, called directly from lifecycle
+  handlers, liveness loop, broadcast, introspect, approve, and reply.
+- **Nascent `AgentBackend` interface** in `agent_backend.go` with `Spawn/Stop/List/Name`. Only
+  used by `handleCreateAgents`. All other code bypasses it.
+- **`AgentUpdate.TmuxSession`** is the field that links an agent to its session. Used across
+  types, DB models, handlers, frontend, scripts, and docs.
+
+### What this design introduces
+
+- **`SessionBackend` interface** with 13 methods covering the full surface: identity
+  (`Name`, `Available`), lifecycle (`CreateSession`, `KillSession`, `SessionExists`,
+  `ListSessions`), status (`GetStatus`), observability (`IsIdle`, `CaptureOutput`,
+  `CheckApproval`), interaction (`SendInput`, `Approve`, `Interrupt`), and discovery
+  (`DiscoverSessions`).
+- **`TmuxSessionBackend`** — pure wrapper around existing tmux functions. Zero behavior change.
+- **`AmbientSessionBackend`** — backed by the ACP public API (`POST /sessions`,
+  `POST /message`, `GET /output`, `POST /stop`, `POST /interrupt`, etc.).
+- **`AgentUpdate.SessionID`** + **`AgentUpdate.BackendType`** — replaces `TmuxSession` with
+  backend-agnostic fields. Backward-compatible JSON unmarshaling for old `tmux_session` payloads.
+- **`Server.backends`** registry — map of backend name to implementation. Agents carry their
+  backend type; the server resolves the right implementation per-agent.
+- **`SessionStatus`** enum — unified status model (`unknown`, `pending`, `running`, `idle`,
+  `completed`, `failed`, `missing`) that both backends map into.
+
+### Interface at a glance
+
+```go
+type SessionBackend interface {
+    Name() string
+    Available() bool
+
+    CreateSession(ctx context.Context, opts SessionCreateOpts) (string, error)
+    KillSession(ctx context.Context, sessionID string) error
+    SessionExists(sessionID string) bool
+    ListSessions() ([]string, error)
+
+    GetStatus(ctx context.Context, sessionID string) (SessionStatus, error)
+
+    IsIdle(sessionID string) bool
+    CaptureOutput(sessionID string, lines int) ([]string, error)
+    CheckApproval(sessionID string) ApprovalInfo
+
+    SendInput(sessionID string, text string) error
+    Approve(sessionID string) error
+    Interrupt(ctx context.Context, sessionID string) error
+
+    DiscoverSessions() (map[string]string, error)
+}
+```
+
+### Scope of changes
+
+| Area | Files affected | Nature of change |
+|------|---------------|-----------------|
+| Interface definition | New: `session_backend.go` | New file |
+| Tmux backend | New: `session_backend_tmux.go` | Wraps existing functions |
+| Old backend | Delete: `agent_backend.go` | Superseded |
+| Tmux primitives | `tmux.go` | Unchanged (kept as unexported helpers) |
+| Data model | `types.go`, `db/models.go`, `db/convert.go`, `db_adapter.go` | Rename `TmuxSession` -> `SessionID`, add `BackendType` |
+| Server | `server.go` | Add `backends` map, `backendFor()` helper |
+| Lifecycle | `lifecycle.go` | Route through backend |
+| Liveness | `liveness.go` | Route through backend |
+| Handlers | `handlers_agent.go` | Route through backend, rename API endpoint |
+| Broadcast | `tmux.go` (orchestration funcs) | Route through backend |
+| Frontend | `types/index.ts`, `AgentDetail.vue`, `client.ts` | Rename `tmux_session` -> `session_id` |
+| Tests | `server_test.go`, `hierarchy_test.go`, `lifecycle_test.go` | Update field names, add mock backend tests |

.spec/SessionBackends/01-tmux-audit.md

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+# Tmux Usage Audit
+
+Every place in the codebase that directly touches tmux, categorized by purpose.
+
+## 1. Low-Level Tmux Primitives (`tmux.go`)
+
+| Function | What it does | Called by |
+|----------|-------------|-----------|
+| `tmuxAvailable()` | Checks if `tmux` binary is in PATH | `TmuxAutoDiscover`, `BroadcastCheckIn`, `SingleAgentCheckIn`, `checkAllSessionLiveness` |
+| `tmuxListSessions()` | Runs `tmux list-sessions -F #S` | `tmuxSessionExists`, `TmuxAutoDiscover` |
+| `tmuxSessionExists(session)` | Checks if a named session is in the list | `handleAgentSpawn`, `handleAgentStop`, `handleAgentRestart`, `handleAgentIntrospect`, `BroadcastCheckIn`, `handleApproveAgent`, `handleReplyAgent`, `handleSpaceTmuxStatus`, `TmuxBackend.Spawn`, `TmuxBackend.Stop`, `checkAllSessionLiveness` |
+| `tmuxCapturePaneLines(session, n)` | Runs `tmux capture-pane -t session -p`, returns last N non-empty lines | `tmuxIsIdle`, `tmuxCheckApproval`, `handleAgentIntrospect`, `handleSpaceTmuxStatus` |
+| `tmuxCapturePaneLastLine(session)` | Wrapper: captures last 1 line | `handleSpaceTmuxStatus` |
+| `tmuxIsIdle(session)` | Checks last 10 lines for idle indicators (shell prompts, Claude Code `>` prompt, etc.) | `tmuxCheckApproval`, `BroadcastCheckIn`, `checkAllSessionLiveness` |
+| `tmuxCheckApproval(session)` | Scans pane for "Do you want...?" + numbered choices pattern | `checkAllSessionLiveness`, `handleAgentIntrospect`, `handleApproveAgent`, `handleSpaceTmuxStatus` |
+| `tmuxApprove(session)` | Sends `Enter` key to session | `handleApproveAgent` |
+| `tmuxSendKeys(session, text)` | Sends text + `C-m` (Enter) to session | `runAgentCheckIn`, `handleAgentSpawn`, `handleAgentRestart`, `handleReplyAgent`, `handleCreateAgents` (ignite), `TmuxBackend.Spawn` |
+| `parseTmuxAgentName(session)` | Extracts agent name from `agentdeck_{name}_{id}` pattern | `TmuxAutoDiscover` |
+
+## 2. Idle Detection Helpers (`tmux.go`)
+
+| Function | What it does |
+|----------|-------------|
+| `lineIsIdleIndicator(line)` | Returns true if a line matches known idle patterns: `>`, shell `$`/`%`/`#`, Claude Code hints, status bars |
+| `isShellPrompt(line)` | Detects `$`, `%`, `>`, `#`, `>` as trailing prompt characters |
+| `waitForIdle(session, timeout)` | Polls `tmuxIsIdle` every 3s until idle or timeout |
+| `waitForBoardPost(space, agent, since, timeout)` | Polls `agentUpdatedAt` every 3s (not tmux-specific, but used exclusively by broadcast which is tmux-only) |
+
+## 3. Broadcast / Check-In (`tmux.go`)
+
+| Function | What it does |
+|----------|-------------|
+| `runAgentCheckIn(space, agent, tmuxSession, checkModel, workModel, result)` | Switches model, sends `/boss.check`, waits for board post, restores model. All via `tmuxSendKeys` + `waitForIdle`. |
+| `BroadcastCheckIn(space, checkModel, workModel)` | Iterates all agents with `TmuxSession`, calls `runAgentCheckIn` concurrently. |
+| `SingleAgentCheckIn(space, agent, checkModel, workModel)` | Single-agent version of broadcast. |
+| `BroadcastResult` + helpers | Result accumulator for sent/skipped/errors. |
+
+## 4. Lifecycle Handlers (`lifecycle.go`)
+
+| Handler | Tmux operations performed |
+|---------|--------------------------|
+| `handleAgentSpawn` | `tmuxSessionExists`, `exec tmux new-session -d`, `tmuxSendKeys` (command), `tmuxSendKeys` (ignite) |
+| `handleAgentStop` | Gets `agent.TmuxSession`, `tmuxSessionExists`, `exec tmux kill-session` |
+| `handleAgentRestart` | Gets `agent.TmuxSession`, `tmuxSessionExists`, `exec tmux kill-session`, `exec tmux new-session`, `tmuxSendKeys` (command + ignite) |
+| `handleAgentIntrospect` | Gets `agent.TmuxSession`, `tmuxSessionExists`, `tmuxIsIdle`, `tmuxCapturePaneLines`, `tmuxCheckApproval` |
+| `isNonTmuxAgent(agent)` | Checks `agent.Registration.AgentType != "tmux"` to gate lifecycle endpoints |
+| `nonTmuxLifecycleError(w, type)` | Returns 422 for non-tmux agents hitting tmux-only endpoints |
+| `inferAgentStatus(exists, idle, needsApproval)` | Pure function mapping booleans to string status (not tmux-specific logic) |
+
+## 5. Liveness Loop (`liveness.go`)
+
+| Function | Tmux operations performed |
+|----------|--------------------------|
+| `checkAllSessionLiveness` | `tmuxAvailable`, iterates all agents with `TmuxSession`, calls `tmuxSessionExists`, `tmuxIsIdle`, `tmuxCheckApproval`. Updates `InferredStatus`, records interrupts, triggers nudges. Broadcasts SSE `tmux_liveness` event. |
+| `executeNudge` | Calls `SingleAgentCheckIn` (which uses tmux) |
+
+## 6. Agent Handlers (`handlers_agent.go`)
+
+| Handler | Tmux operations performed |
+|---------|--------------------------|
+| `handleSpaceAgent` (POST) | Preserves `TmuxSession` as sticky field on agent update |
+| `handleIgnition` (GET) | Accepts `?tmux_session=` query param, stores on agent record, references it in ignition text |
+| `handleSpaceTmuxStatus` (GET) | `TmuxAutoDiscover`, iterates agents, calls `tmuxSessionExists`, `tmuxIsIdle`, `tmuxCapturePaneLastLine`, `tmuxCheckApproval` |
+| `handleApproveAgent` (POST) | Gets `agent.TmuxSession`, `tmuxSessionExists`, `tmuxCheckApproval`, `tmuxApprove` |
+| `handleReplyAgent` (POST) | Gets `agent.TmuxSession`, `tmuxSessionExists`, `tmuxSendKeys` |
+| `handleCreateAgents` (POST) | Uses `AgentBackend` interface for spawn, but then calls `tmuxSendKeys` directly for ignite |
+
+## 7. Backend Interface (`agent_backend.go`) — already exists but incomplete
+
+| Method | `TmuxBackend` impl | `CloudBackend` impl |
+|--------|-------------------|---------------------|
+| `Name()` | `"tmux"` | `"cloud"` |
+| `Spawn(ctx, spec)` | Creates tmux session, sends command | Returns `ErrNotImplemented` |
+| `Stop(ctx, space, name)` | Kills tmux session | Returns `ErrNotImplemented` |
+| `List(ctx, space)` | Lists all tmux sessions | Returns `ErrNotImplemented` |
+
+**Only used by:** `handleCreateAgents`. All other lifecycle/liveness/broadcast code bypasses this interface entirely.
+
+## 8. Data Model References
+
+| Location | Field | Notes |
+|----------|-------|-------|
+| `types.go:96` | `AgentUpdate.TmuxSession` | JSON tag `tmux_session` |
+| `db/models.go:82` | `Agent.TmuxSession` | SQLite column |
+| `db/convert.go:35` | `AgentRow.TmuxSession` | DB-to-coordinator conversion |
+| `db/convert.go:61,113` | `FromAgentFields(..., tmuxSession, ...)` | Coordinator-to-DB conversion |
+| `db_adapter.go:317,371` | References `TmuxSession` | DB adapter layer |
+| `db/migrate_from_json.go:40` | `jsonAgent.TmuxSession` | JSON migration |
+
+## 9. Frontend References
+
+| File | Usage |
+|------|-------|
+| `frontend/src/types/index.ts:49,118` | `tmux_session?: string` on agent types |
+| `frontend/src/api/client.ts:257,275` | Spawn/restart return `tmux_session` |
+| `frontend/src/components/AgentDetail.vue:133,562,918,921` | Displays tmux session, gates pane/controls sections |
+
+## 10. Scripts and Documentation
+
+| File | Usage |
+|------|-------|
+| `scripts/boss.sh` | `get_tmux_session()`, passes `-e TMUX_SESSION` |
+| `scripts/agent-ignition.sh` | `create_tmux_session()` |
+| `scripts/coordination-client.py` | Passes `tmux_session` to ignition |
+| `commands/boss.ignite.md` | References `?tmux_session=` |
+| `commands/boss.check.md` | Notes `tmux_session` is sticky |
+| `docs/AGENT_PROTOCOL.md` | Documents `tmux_session` field, ignition params |
+| `docs/lifecycle-spec.md` | Spawn/stop/restart reference `tmux_session` |
+| `docs/api-reference.md` | API docs reference `tmux_session` |
+| `docs/hierarchy-design.md` | Compares parent stickiness to `TmuxSession` |