T-107: Remove redundant test coverage

[jaesbit]

Summary

• Audited the test suite and removed or merged 10 redundant or low-value checks while retaining stronger neighboring coverage.
• Updated stale modelByDifficulty fixtures in Auto Work database and scheduler tests.
• Fixed the three previously failing test cases:
  • The KB index invalidation assertion now checks the rebuilt graph's membership rather than an undocumented directory traversal order.
  • The InProcess queue harness now provides sessionManager.getHeader and planBridge.getPendingPlanExecution, matching the current snapshot() contract.

Verification

• bun test
  • 1079 pass, 0 fail
  • 2796 expect() calls
  • 98 files