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)

gardener/dust.jsonl

@@ -0,0 +1 @@
+{"issue":915,"group":"lib/generators.sh","title":"remove no-op sed in generate_compose --build mode","reason":"sed replaces agents: with itself — no behavior change; single-line removal","ts":"2026-04-17T01:04:05Z"}