chore: gardener housekeeping 2026-04-17

- Promote #910, #914, #867 to backlog with acceptance criteria + affected files
- Promote #820 to backlog (already well-structured, dep on #758 gates pickup)
- Stage #915 as dust (no-op sed, single-line removal)
- Update all AGENTS.md watermarks to HEAD
- Root AGENTS.md: document vault-seed-<svc>.sh convention + complete test file list
- Track gardener/dust.jsonl in git (remove from .gitignore)

.gitignore

@@ -20,7 +20,6 @@ metrics/supervisor-metrics.jsonl
 # OS
 .DS_Store
 dev/ci-fixes-*.json
-gardener/dust.jsonl
 
 # Individual encrypted secrets (managed by disinto secrets add)
 secrets/