feat(routing): configurable default responder for new threads (#385)

docs/designs/issue-385-default-responder.md

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+# Issue #385: Configurable Default Responder for New Threads
+
+Parent: F127 (Member Overview Refactor)
+
+## Problem
+
+The "global default responder" for new threads (no history, no @mention, no preferredCats) is currently derived from `breeds[0].defaultVariantId`, hardcoded to resolve to `opus`. This couples the default to breed order rather than an explicit runtime-configurable choice.
+
+## Design
+
+### Member Overview Card (Proposed)
+
+- Default responder cat gets a `star + "默认回复"` orange badge next to its name
+- Default cat card border: 2px `#D49266` (vs 1px `#F1E7DF` for non-default)
+- Non-default cats: unchanged appearance
+
+### Member Detail Panel (New Toggle)
+
+- **Label**: "全局默认回复猫"
+- **Description**: "仅影响新会话无历史时第一条消息，不影响已有 thread 的续接逻辑"
+- **Toggle**: standard on/off
+- **Info box** (when toggle is off and another cat is default): "当前默认: {name}（{catId}）。设为此成员后，{name}的默认状态将自动取消。若此成员不可用（available=false），将退回 breed 默认。"
+
+### Visual reference
+
+Pencil mockup: `/pencil-new.pen` (3 frames: Current State, Proposed State, Member Detail Panel)
+
+## Technical Plan
+
+### Storage
+
+- Add `defaultResponderCatId?: CatId` at the **catalog top level** (CatCafeConfig)
+- NOT a boolean on each RosterEntry (avoids mutual exclusion complexity)
+- Single atomic write: set B = clear A implicitly
+
+### New Resolver
+
+- Create `getDefaultResponderCatId()` — dedicated to "new thread no-history" semantics
+- Keep existing `getDefaultCatId()` unchanged — reviewer fallback etc. stay as-is
+- Fallback chain: `defaultResponderCatId` (roster) -> `breeds[0].defaultVariantId` (breed) -> `'opus'` (hardcoded)
+
+### Affected Call Sites
+
+| Location | Current | Change |
+|----------|---------|--------|
+| `AgentRouter.ts` final fallback | `getDefaultCatId()` | `getDefaultResponderCatId()` |
+| `ConnectorRouter.ts:277,373` | frozen `'opus'` | dynamic `getDefaultResponderCatId()` |
+| `index.ts:1973` | `defaultCatId: 'opus'` | dynamic read |
+| `ChatContainer.tsx:486` | `targetCats[0] \|\| 'opus'` | fetch from config |
+| `reviewer-matcher.ts:72` | `getDefaultCatId()` | **NO CHANGE** (stays as-is) |
+
+### API
+
+- `PATCH /api/config` with `{ key: 'defaultResponderCatId', value: catId }` — reuses existing config patch endpoint
+- Uniqueness enforced: single value, not per-member boolean
+
+### Verification Checklist
+
+- [x] Hub member overview: switch default -> refresh -> still shows correct member
+- [x] New thread / no history / no @mention -> routes to configured default
+- [x] Connector new external thread -> uses configured default, not frozen 'opus'
+- [x] Default member unavailable (available=false) -> falls back to breed default
+- [x] Reviewer fallback semantics NOT changed
+
+## Scope Exclusions
+
+- preferredCats semantics: unchanged
+- Last-replier logic for existing threads: unchanged
+- routingPolicy: unchanged
+- Reviewer fallback: unchanged (stays on `getDefaultCatId()`)