feat(phase5): add usage, feedback, annotations, bulk delete (v0.6.4)

Phase 5 New Capabilities:

1. nia_usage - Quota reporting tool
   - GET /usage endpoint
   - Shows credits used/remaining, reset date, plan

2. nia_feedback - Feedback submission tool
   - POST /feedback/answer - Rate answers
   - POST /feedback/source - Rate sources
   - POST /feedback/interaction - Interaction signals

3. Source Annotations - Added to nia_manage_resource
   - annotation_create - POST /sources/{id}/annotations
   - annotation_list - GET /sources/{id}/annotations
   - annotation_delete - DELETE /sources/{id}/annotations

4. Bulk Delete - Added to nia_manage_resource
   - bulk_delete action - POST /sources/bulk-delete
   - Permission prompt before execution

All features include:
- Unit tests (26 new tests)
- Integration tests (gated on NIA_API_KEY)
- Config flags (default enabled)
- README documentation

Build passes, all tests pass.