Subagent context compaction policy

[TroyHernandez]

Summary

Working subagents now have a configurable in-memory history compaction policy. The on-disk transcript (from PR #75) is untouched — only the live context sent to the model is rewritten.

Config

```yaml
subagents:
context_compaction:
mode: inherit_strict # inherit_strict | inherit | off
compact_pct: 75 # child threshold (percent of model limit)
keep_recent_turns: 1 # turns preserved verbatim after compact
min_messages: 6 # below this, never compact
timeout_seconds: 60 # hard wall on the summarizer call
```

Modes

• `inherit_strict` (default): effective threshold = `min(parent.context_compact_pct, child.compact_pct)`. Async children can only be equal-or-lower than the parent — they shouldn't silently outgrow the parent's tolerance.
• `inherit`: use the parent's `context_compact_pct` verbatim.
• `off`: never compact this child.

Flow

`maybe_compact_turn_session` runs after the turn finishes (never mid-turn) and after archival has had its shot:

1. Skip if `mode == off` or `kind == "archive_holder"` (seed-history-only subagents must not lose their transcript).
2. Skip if history shorter than `min_messages`.
3. Measure usage via `context_usage_pct` (from PR Centralize context-budget helpers into package code #76). Below threshold → skip.
4. `compact_find_cut` walks back to the largest safe cut that:
   • Keeps the most recent `keep_recent_turns` turns verbatim.
   • Never splits a `tool_use` from its `tool_result` (LLM would reject a dangling tool_use).
5. `compact_summarize_slice` asks the LLM to summarize the prefix under a `setTimeLimit`.
6. On success: `session$history` becomes `[ "[compacted history]\n\n
   " ] + kept_tail`.
7. On any failure (no safe cut, summarizer error, timeout): log and leave history intact.

Archive-holder protection

`subagent_seed_history` now stamps `.subagent_state$kind <- "archive_holder"`. The compaction hook sees that and bails immediately, so an archived-turn holder never loses its seeded transcript.

Tests

• 24 offline tests in `test_compact_turn.R`: threshold resolution (all three modes), the cut-point finder (including the open-tool-use case), the pure history rewrite, and the early-bail paths.
• `tinytest::test_package("corteza")` — 1510/1510 OK.
• End-to-end smoke against moonshot: spawn → query → hook runs as a no-op below `min_messages`, transcript stays intact, no errors.

Stack

• PR Persist working subagent transcripts to disk #75 (durable subagent transcripts) — already merged.
• PR Centralize context-budget helpers into package code #76 (centralize context-budget helpers) — already merged.
• This PR completes the codex-proposed subagent followups for compaction.