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

gardener/AGENTS.md

@@ -0,0 +1,23 @@
+<!-- last-reviewed: 16e430ebce905cf722f91ae1f873faea1c64d890 -->
+# Gardener Agent
+
+**Role**: Backlog grooming — detect duplicate issues, missing acceptance
+criteria, oversized issues, stale issues, and circular dependencies. Invoke
+Claude to fix or escalate to a human via Matrix.
+
+**Trigger**: `gardener-run.sh` runs 2x/day via cron. It files an `action`
+issue referencing `formulas/run-gardener.toml`; the action-agent picks it up
+and executes the gardener steps in an interactive Claude tmux session. Accepts
+an optional project TOML argument (configures which project the action issue is
+filed against).
+
+**Key files**:
+- `gardener/gardener-run.sh` — Cron wrapper: lock, memory guard, dedup check, files action issue
+- `gardener/gardener-poll.sh` — Escalation-reply injection for dev sessions, invokes gardener-agent.sh for grooming
+- `gardener/gardener-agent.sh` — Orchestrator: bash pre-analysis, creates tmux session (`gardener-{project}`) with interactive `claude`, monitors phase file, parses result file (ACTION:/DUST:/ESCALATE)
+- `formulas/run-gardener.toml` — Execution spec: preflight, grooming, dust-bundling, blocked-review, agents-update, commit-and-pr
+
+**Environment variables consumed**:
+- `CODEBERG_TOKEN`, `CODEBERG_REPO`, `CODEBERG_API`, `PROJECT_NAME`, `PROJECT_REPO_ROOT`
+- `CLAUDE_TIMEOUT`
+- `MATRIX_TOKEN`, `MATRIX_ROOM_ID`, `MATRIX_HOMESERVER`