Fix storage backend extensibility by adding DeleteIssue to Storage interface

- Added DeleteIssue to Storage interface
- Implemented DeleteIssue in MemoryStorage backend
- Removed brittle type assertion from deletion_tracking.go
- Closes bd-1fkr

internal/storage/storage.go

@@ -17,6 +17,7 @@ type Storage interface {
 	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
+	DeleteIssue(ctx context.Context, id string) error
 	SearchIssues(ctx context.Context, query string, filter types.IssueFilter) ([]*types.Issue, error)
 
 	// Dependencies