feat(session): add CacheUpdateStatus to ExecutionEventKind::Finish

Add CacheUpdateStatus enum to track whether cache was updated after task
execution and why. This provides visibility into cache behavior in the
Finish event.

- Add CacheNotUpdatedReason enum with CacheHit, CacheDisabled, NonZeroExitStatus
- Add CacheUpdateStatus enum with Updated and NotUpdated variants
- Include cache_update_status in ExecutionEventKind::Finish
- Update all Finish event emissions to include appropriate status

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: branchseer