Implement external_ref as primary matching key for import updates (bd-1022)

- 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

internal/storage/sqlite/schema.go

@@ -30,6 +30,7 @@ CREATE INDEX IF NOT EXISTS idx_issues_status ON issues(status);
 CREATE INDEX IF NOT EXISTS idx_issues_priority ON issues(priority);
 CREATE INDEX IF NOT EXISTS idx_issues_assignee ON issues(assignee);
 CREATE INDEX IF NOT EXISTS idx_issues_created_at ON issues(created_at);
+CREATE INDEX IF NOT EXISTS idx_issues_external_ref ON issues(external_ref);
 
 -- Dependencies table
 CREATE TABLE IF NOT EXISTS dependencies (