chore: gardener housekeeping 2026-03-21

Progressive disclosure split of AGENTS.md (487→152 lines):
- Extracted per-directory AGENTS.md files for all 8 agents + lib/
- Root AGENTS.md now serves as a table of contents with summary table
- All watermarks updated to 16e430e

Grooming results:
- Promoted #469 (WATCH flow missing curl) and #436 (idle_pane_count bug) to backlog
- 12 dust items classified, no groups ripe for bundling yet
- No blocked issues, no AD violations

action/AGENTS.md

@@ -0,0 +1,31 @@
+<!-- last-reviewed: 16e430ebce905cf722f91ae1f873faea1c64d890 -->
+# Action Agent
+
+**Role**: Execute operational tasks described by action formulas — run scripts,
+call APIs, send messages, collect human approval. Shares the same phase handler
+as the dev-agent: if an action produces code changes, the orchestrator creates a
+PR and drives the CI/review loop; otherwise Claude closes the issue directly.
+
+**Trigger**: `action-poll.sh` runs every 10 min via cron. It scans for open
+issues labeled `action` that have no active tmux session, then spawns
+`action-agent.sh <issue-number>`.
+
+**Key files**:
+- `action/action-poll.sh` — Cron scheduler: finds open action issues with no active tmux session, spawns action-agent.sh
+- `action/action-agent.sh` — Orchestrator: fetches issue body + prior comments, creates tmux session (`action-{issue_num}`) with interactive `claude`, injects formula prompt with phase protocol, enters `monitor_phase_loop` (shared via `dev/phase-handler.sh`) for CI/review lifecycle or direct completion
+
+**Session lifecycle**:
+1. `action-poll.sh` finds open `action` issues with no active tmux session.
+2. Spawns `action-agent.sh <issue_num>`.
+3. Agent creates Matrix thread, exports `MATRIX_THREAD_ID` so Claude's output streams to the thread via a Stop hook (`on-stop-matrix.sh`).
+4. Agent creates tmux session `action-{issue_num}`, injects prompt (formula + prior comments + phase protocol).
+5. Agent enters `monitor_phase_loop` (shared with dev-agent via `dev/phase-handler.sh`).
+6. **Path A (git output):** Claude pushes branch → `PHASE:awaiting_ci` → handler creates PR, polls CI → injects failures → Claude fixes → push → re-poll → CI passes → `PHASE:awaiting_review` → handler polls reviews → injects REQUEST_CHANGES → Claude fixes → approved → merge → cleanup.
+7. **Path B (no git output):** Claude posts results as comment, closes issue → `PHASE:done` → handler cleans up (kill session, docker compose down, remove temp files).
+8. For human input: Claude sends a Matrix message and waits; the reply is injected into the session by `matrix_listener.sh`.
+
+**Environment variables consumed**:
+- `CODEBERG_TOKEN`, `CODEBERG_REPO`, `CODEBERG_API`, `PROJECT_NAME`, `CODEBERG_WEB`
+- `MATRIX_TOKEN`, `MATRIX_ROOM_ID`, `MATRIX_HOMESERVER` — Matrix notifications + human input
+- `ACTION_IDLE_TIMEOUT` — Max seconds before killing idle session (default 14400 = 4h)
+- `ACTION_MAX_LIFETIME` — Max total session wall-clock seconds (default 28800 = 8h); caps session independently of idle timeout