Add auto-migration for dirty_issues table

Implement automatic database migration to add the dirty_issues table
for existing databases that were created before the incremental export
feature (bd-39) was implemented.

Changes:
- Add migrateDirtyIssuesTable() function in sqlite.go
- Check for dirty_issues table existence on database initialization
- Create table and index if missing (silent migration)
- Call migration after schema initialization in New()

The migration:
- Queries sqlite_master to check if dirty_issues table exists
- If missing, creates the table with proper schema and index
- Happens automatically on first database access after upgrade
- No user intervention required
- Fails safely if table already exists (no-op)

Testing:
- Created test database without dirty_issues table
- Verified table was auto-created on first command
- Verified issue was properly marked dirty
- All existing tests pass

This makes the incremental export feature (bd-39) work seamlessly
with existing databases without requiring manual migration steps.

Closes bd-51

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/sqlite.go

@@ -48,6 +48,11 @@ func New(path string) (*SQLiteStorage, error) {
 		return nil, fmt.Errorf("failed to initialize schema: %w", err)
 	}
 
+	// Migrate existing databases to add dirty_issues table if missing
+	if err := migrateDirtyIssuesTable(db); err != nil {
+		return nil, fmt.Errorf("failed to migrate dirty_issues table: %w", err)
+	}
+
 	// Get next ID
 	nextID := getNextID(db)
 
@@ -57,6 +62,41 @@ func New(path string) (*SQLiteStorage, error) {
 	}, nil
 }
 
+// migrateDirtyIssuesTable checks if the dirty_issues table exists and creates it if missing.
+// This ensures existing databases created before the incremental export feature get migrated automatically.
+func migrateDirtyIssuesTable(db *sql.DB) error {
+	// Check if dirty_issues table exists
+	var tableName string
+	err := db.QueryRow(`
+		SELECT name FROM sqlite_master
+		WHERE type='table' AND name='dirty_issues'
+	`).Scan(&tableName)
+
+	if err == sql.ErrNoRows {
+		// Table doesn't exist, create it
+		_, err := db.Exec(`
+			CREATE TABLE dirty_issues (
+				issue_id TEXT PRIMARY KEY,
+				marked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+				FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+			);
+			CREATE INDEX idx_dirty_issues_marked_at ON dirty_issues(marked_at);
+		`)
+		if err != nil {
+			return fmt.Errorf("failed to create dirty_issues table: %w", err)
+		}
+		// Table created successfully - no need to log, happens silently
+		return nil
+	}
+
+	if err != nil {
+		return fmt.Errorf("failed to check for dirty_issues table: %w", err)
+	}
+
+	// Table exists, no migration needed
+	return nil
+}
+
 // getNextID determines the next issue ID to use
 func getNextID(db *sql.DB) int {
 	// Get prefix from config, default to "bd"