Add /compact skill for manual context compaction (#817)

* feat: add /compact skill for manual context compaction

added /compact session command to fight context rot in long-running sessions. Uses Claude Agent SDK's built-in /compact command with auth gating (main-group or is_from_me only).

* simplify: remove group-queue modification, streamline denied path confirmed against fresh-clone merge.

* refactor: extract handleSessionCommand from index.ts into session-commands.ts

Verified: 345/345 tests pass on fresh-clone merge.

.claude/skills/add-compact/modify/container/agent-runner/src/index.ts.intent.md

@@ -0,0 +1,29 @@
+# Intent: container/agent-runner/src/index.ts
+
+## What Changed
+- Added `KNOWN_SESSION_COMMANDS` whitelist (`/compact`)
+- Added slash command handling block in `main()` between prompt building and query loop
+- Slash commands use `query()` with string prompt (not MessageStream), `allowedTools: []`, no mcpServers
+- Tracks `compactBoundarySeen`, `hadError`, `resultEmitted` flags
+- Observes `compact_boundary` system event to confirm compaction
+- PreCompact hook still registered for transcript archival
+- Error subtype checking: `resultSubtype?.startsWith('error')` emits `status: 'error'`
+- Container exits after slash command completes (no IPC wait loop)
+
+## Key Sections
+- **KNOWN_SESSION_COMMANDS** (before query loop): Set containing `/compact`
+- **Slash command block** (after prompt building, before query loop): Detects session command, runs query with minimal options, handles result/error/boundary events
+- **Existing query loop**: Unchanged
+
+## Invariants (must-keep)
+- ContainerInput/ContainerOutput interfaces
+- readStdin, writeOutput, log utilities
+- OUTPUT_START_MARKER / OUTPUT_END_MARKER protocol
+- MessageStream class with push/end/asyncIterator
+- IPC polling (drainIpcInput, waitForIpcMessage, shouldClose)
+- runQuery function with all existing logic
+- createPreCompactHook for transcript archival
+- createSanitizeBashHook for secret stripping
+- parseTranscript, formatTranscriptMarkdown helpers
+- main() stdin parsing, SDK env setup, query loop
+- SECRET_ENV_VARS list