Resolves#23
- Replace per-dependency `AgentPhase::find()` loop with a single
`AgentPhase::whereIn('id', $dependencies)->get()` call, reducing
query count from N to 1 for any number of dependencies
- Short-circuit early when dependencies list is empty to avoid
unnecessary query at all
- Add tests: empty deps, skipped-dep passthrough, single-query
assertion, blocker shape validation
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>