T036-T038: Complete ClusterDetector test coverage

Unit tests (T036-T037):
- _determine_action(): MERGE/LINK/IGNORE thresholds
- _calculate_confidence(): size factors, cohesion weighting
- _cluster_key(): sorted key generation
- Initialization with custom parameters
- process_item() using cached values

Integration test (T038):
- Full cluster workflow from scan to results
- Action based on cohesion thresholds
- Dry-run mode (no mutations)
- Edge cases: empty storage, single memory, no overlap
- Custom threshold and min_cluster_size

Test counts: 19 unit + 13 integration = 32 new tests
Full suite: 1192 passed, 7 skipped

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: prefrontalsys