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/manifest.yaml

@@ -0,0 +1,16 @@
+skill: add-compact
+version: 1.0.0
+description: "Add /compact command for manual context compaction via Claude Agent SDK"
+core_version: 1.2.10
+adds:
+  - src/session-commands.ts
+  - src/session-commands.test.ts
+modifies:
+  - src/index.ts
+  - container/agent-runner/src/index.ts
+structured:
+  npm_dependencies: {}
+  env_additions: []
+conflicts: []
+depends: []
+test: "npx vitest run --config vitest.skills.config.ts .claude/skills/add-compact/tests/add-compact.test.ts"