feat: minimal backend testing infrastructure with integration test

- Created working integration test for POST /run/start
  - Tests complete flow: start → worker → agent → graph projection
  - Verifies uniqueScreensDiscovered >= 1
  - Runs in encore test (no encore run needed)
  - Duration: ~15 seconds

- Added service registration (backend/agent/encore.service.ts)
  - Loads worker subscription
  - Critical for PubSub in encore test

- Added graph query helpers (backend/graph/queries.ts)
  - countUniqueScreensDiscovered()
  - getUniqueScreensDiscovered()
  - getRunStatus()

- Fixed vitest config for path aliases
  - Added ~encore alias to resolve encore.gen
  - Enables import of generated clients in tests

- Updated skills
  - backend-testing: Focused on integration testing pattern
  - backend-debugging: Simplified with practical queries
  - Removed petty testing patterns

- Removed unnecessary code
  - Deleted 8 files (petty unit tests, docs, unused endpoints)
  - Net: -1248 lines of unnecessary code

Testing is now the backbone for success in this project.

Author: nirukk52