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

review/AGENTS.md

@@ -0,0 +1,20 @@
+<!-- last-reviewed: 16e430ebce905cf722f91ae1f873faea1c64d890 -->
+# Review Agent
+
+**Role**: AI-powered PR review — post structured findings and formal
+approve/request-changes verdicts to Codeberg.
+
+**Trigger**: `review-poll.sh` runs every 10 min via cron. It scans open PRs
+whose CI has passed and that lack a review for the current HEAD SHA, then
+spawns `review-pr.sh <pr-number>`.
+
+**Key files**:
+- `review/review-poll.sh` — Cron scheduler: finds unreviewed PRs with passing CI
+- `review/review-pr.sh` — Creates/reuses a tmux session (`review-{project}-{pr}`), injects PR diff, waits for Claude to write structured JSON output, posts markdown review + formal Codeberg review, auto-creates follow-up issues for pre-existing tech debt
+
+**Environment variables consumed**:
+- `CODEBERG_TOKEN` — Dev-agent token (must not be the same account as REVIEW_BOT_TOKEN)
+- `REVIEW_BOT_TOKEN` — Review-agent token for approvals (use human/admin account; branch protection: in approvals whitelist)
+- `CODEBERG_REPO`, `CODEBERG_API`, `PROJECT_NAME`, `PROJECT_REPO_ROOT`
+- `PRIMARY_BRANCH`, `WOODPECKER_REPO_ID`
+- `MATRIX_TOKEN`, `MATRIX_ROOM_ID`, `MATRIX_HOMESERVER`