test: Add comprehensive tests for org-agenda cacheperformance

Add 9 ERT tests covering org-agenda file list caching:

Normal cases:
- First call builds cache from scratch
- Second call uses cached files (validates cache hit)
- Force rebuild bypasses valid cache

Boundary cases:
- Cache expires after TTL and triggers rebuild
- Empty directories create minimal base file list
- Building flag set during build and cleared after
- Building flag clears on error (unwind-protect)

Error cases:
- Nil cache with timestamp triggers rebuild
- Directory scan failures propagate correctly

All tests pass (9/9). No regressions in full suite.
Total: 1,823 tests, 17 pre-existing failures (down from 18).

0 files changed, 0 insertions, 0 deletions