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

vault/AGENTS.md

@@ -0,0 +1,20 @@
+<!-- last-reviewed: 16e430ebce905cf722f91ae1f873faea1c64d890 -->
+# Vault Agent
+
+**Role**: Safety gate for dangerous or irreversible actions. Actions enter a
+pending queue and are classified by Claude via `vault-agent.sh`, which can
+auto-approve (call `vault-fire.sh` directly), auto-reject (call
+`vault-reject.sh`), or escalate to a human via Matrix for APPROVE/REJECT.
+
+**Trigger**: `vault-poll.sh` runs every 30 min via cron.
+
+**Key files**:
+- `vault/vault-poll.sh` — Processes pending actions: retry approved, auto-reject after 48h timeout, invoke vault-agent for new items
+- `vault/vault-agent.sh` — Classifies and routes pending actions via `claude -p`: auto-approve, auto-reject, or escalate to human
+- `vault/PROMPT.md` — System prompt for the vault agent's Claude invocation
+- `vault/vault-fire.sh` — Executes an approved action
+- `vault/vault-reject.sh` — Marks an action as rejected
+
+**Environment variables consumed**:
+- All from `lib/env.sh`
+- `MATRIX_TOKEN`, `MATRIX_ROOM_ID`, `MATRIX_HOMESERVER` — Escalation channel