- Add GetIssueByExternalRef() query function to storage interface and implementations
- Update DetectCollisions() to prioritize external_ref matching over ID matching
- Modify upsertIssues() to handle external_ref matches in import logic
- Add index on external_ref column for performance
- Add comprehensive tests for external_ref matching in both collision detection and import
- Enables re-syncing from external systems (Jira, GitHub, Linear) without duplicates
- Preserves local issues (no external_ref) from being overwritten
