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/storage.go

@@ -14,6 +14,7 @@ type Storage interface {
 	CreateIssue(ctx context.Context, issue *types.Issue, actor string) error
 	CreateIssues(ctx context.Context, issues []*types.Issue, actor string) error
 	GetIssue(ctx context.Context, id string) (*types.Issue, error)
+	GetIssueByExternalRef(ctx context.Context, externalRef string) (*types.Issue, error)
 	UpdateIssue(ctx context.Context, id string, updates map[string]interface{}, actor string) error
 	CloseIssue(ctx context.Context, id string, reason string, actor string) error
 	SearchIssues(ctx context.Context, query string, filter types.IssueFilter) ([]*types.Issue, error)