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

cmd/bd/deletion_tracking.go

@@ -99,20 +99,10 @@ func merge3WayAndPruneDeletions(ctx context.Context, store storage.Storage, json
 	}
 
 	// Prune accepted deletions from the database
-	// Use type assertion to access DeleteIssue method (available in concrete SQLiteStorage)
-	type deleter interface {
-		DeleteIssue(context.Context, string) error
-	}
-
-	d, ok := store.(deleter)
-	if !ok {
-		return false, fmt.Errorf("storage backend does not support DeleteIssue")
-	}
-
 	// Collect all deletion errors - fail the operation if any delete fails
 	var deletionErrors []error
 	for _, id := range acceptedDeletions {
-		if err := d.DeleteIssue(ctx, id); err != nil {
+		if err := store.DeleteIssue(ctx, id); err != nil {
 			deletionErrors = append(deletionErrors, fmt.Errorf("issue %s: %w", id, err))
 		}
 	}

internal/storage/memory/memory.go

@@ -406,6 +406,29 @@ func (m *MemoryStorage) CloseIssue(ctx context.Context, id string, reason string
 	}, actor)
 }
 
+// DeleteIssue permanently deletes an issue and all associated data
+func (m *MemoryStorage) DeleteIssue(ctx context.Context, id string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Check if issue exists
+	if _, ok := m.issues[id]; !ok {
+		return fmt.Errorf("issue not found: %s", id)
+	}
+
+	// Delete the issue
+	delete(m.issues, id)
+
+	// Delete associated data
+	delete(m.dependencies, id)
+	delete(m.labels, id)
+	delete(m.events, id)
+	delete(m.comments, id)
+	delete(m.dirty, id)
+
+	return nil
+}
+
 // SearchIssues finds issues matching query and filters
 func (m *MemoryStorage) SearchIssues(ctx context.Context, query string, filter types.IssueFilter) ([]*types.Issue, error) {
 	m.mu.RLock()

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