Add comments feature (bd-162)

- Add comments table to SQLite schema
- Add Comment type to internal/types
- Implement AddIssueComment and GetIssueComments in storage layer
- Update JSONL export/import to include comments
- Add comments to 'bd show' output
- Create 'bd comments' CLI command structure
- Fix UpdateIssueID to update comments table and defer FK checks
- Add GetIssueComments/AddIssueComment to Storage interface

Note: CLI command needs daemon RPC support (tracked in bd-163)
Amp-Thread-ID: https://ampcode.com/threads/T-ece10dd1-cf64-48ff-9adb-dd304d0bcb25
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -1155,6 +1155,12 @@ func (s *SQLiteStorage) UpdateIssueID(ctx context.Context, oldID, newID string,
 	}
 	defer tx.Rollback()
 
+	// Defer foreign key checks until end of transaction
+	_, err = tx.ExecContext(ctx, `PRAGMA defer_foreign_keys = ON`)
+	if err != nil {
+		return fmt.Errorf("failed to defer foreign keys: %w", err)
+	}
+
 	_, err = tx.ExecContext(ctx, `
 		UPDATE issues
 		SET id = ?, title = ?, description = ?, design = ?, acceptance_criteria = ?, notes = ?, updated_at = ?
@@ -1184,6 +1190,11 @@ func (s *SQLiteStorage) UpdateIssueID(ctx context.Context, oldID, newID string,
 		return fmt.Errorf("failed to update labels: %w", err)
 	}
 
+	_, err = tx.ExecContext(ctx, `UPDATE comments SET issue_id = ? WHERE issue_id = ?`, newID, oldID)
+	if err != nil {
+		return fmt.Errorf("failed to update comments: %w", err)
+	}
+
 	_, err = tx.ExecContext(ctx, `
 		UPDATE dirty_issues SET issue_id = ? WHERE issue_id = ?
 	`, newID, oldID)
@@ -1716,6 +1727,81 @@ func (s *SQLiteStorage) GetMetadata(ctx context.Context, key string) (string, er
 	return value, err
 }
 
+// AddIssueComment adds a comment to an issue
+func (s *SQLiteStorage) AddIssueComment(ctx context.Context, issueID, author, text string) (*types.Comment, error) {
+	// Verify issue exists
+	var exists bool
+	err := s.db.QueryRowContext(ctx, `SELECT EXISTS(SELECT 1 FROM issues WHERE id = ?)`, issueID).Scan(&exists)
+	if err != nil {
+		return nil, fmt.Errorf("failed to check issue existence: %w", err)
+	}
+	if !exists {
+		return nil, fmt.Errorf("issue %s not found", issueID)
+	}
+
+	// Insert comment
+	result, err := s.db.ExecContext(ctx, `
+		INSERT INTO comments (issue_id, author, text, created_at)
+		VALUES (?, ?, ?, CURRENT_TIMESTAMP)
+	`, issueID, author, text)
+	if err != nil {
+		return nil, fmt.Errorf("failed to insert comment: %w", err)
+	}
+
+	// Get the inserted comment ID
+	commentID, err := result.LastInsertId()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get comment ID: %w", err)
+	}
+
+	// Fetch the complete comment
+	comment := &types.Comment{}
+	err = s.db.QueryRowContext(ctx, `
+		SELECT id, issue_id, author, text, created_at
+		FROM comments WHERE id = ?
+	`, commentID).Scan(&comment.ID, &comment.IssueID, &comment.Author, &comment.Text, &comment.CreatedAt)
+	if err != nil {
+		return nil, fmt.Errorf("failed to fetch comment: %w", err)
+	}
+
+	// Mark issue as dirty for JSONL export
+	if err := s.MarkIssueDirty(ctx, issueID); err != nil {
+		return nil, fmt.Errorf("failed to mark issue dirty: %w", err)
+	}
+
+	return comment, nil
+}
+
+// GetIssueComments retrieves all comments for an issue
+func (s *SQLiteStorage) GetIssueComments(ctx context.Context, issueID string) ([]*types.Comment, error) {
+	rows, err := s.db.QueryContext(ctx, `
+		SELECT id, issue_id, author, text, created_at
+		FROM comments
+		WHERE issue_id = ?
+		ORDER BY created_at ASC
+	`, issueID)
+	if err != nil {
+		return nil, fmt.Errorf("failed to query comments: %w", err)
+	}
+	defer rows.Close()
+
+	var comments []*types.Comment
+	for rows.Next() {
+		comment := &types.Comment{}
+		err := rows.Scan(&comment.ID, &comment.IssueID, &comment.Author, &comment.Text, &comment.CreatedAt)
+		if err != nil {
+			return nil, fmt.Errorf("failed to scan comment: %w", err)
+		}
+		comments = append(comments, comment)
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("error iterating comments: %w", err)
+	}
+
+	return comments, nil
+}
+
 // Close closes the database connection
 func (s *SQLiteStorage) Close() error {
 	return s.db.Close()