feat(storage): add Dolt backend for version-controlled issue storage

Implements a complete Dolt storage backend that mirrors the SQLite implementation
with MySQL-compatible syntax and adds version control capabilities.

Key features:
- Full Storage interface implementation (~50 methods)
- Version control operations: commit, push, pull, branch, merge, checkout
- History queries via AS OF and dolt_history_* tables
- Cell-level merge instead of line-level JSONL merge
- SQL injection protection with input validation

Bug fixes applied during implementation:
- Added missing quality_score, work_type, source_system to scanIssue
- Fixed Status() to properly parse boolean staged column
- Added validation to CreateIssues (was missing in batch create)
- Made RenameDependencyPrefix transactional
- Expanded GetIssueHistory to return more complete data

Test coverage: 17 tests covering CRUD, dependencies, labels, search,
comments, events, statistics, and SQL injection protection.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/labels.go

@@ -0,0 +1,118 @@
+package dolt
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// AddLabel adds a label to an issue
+func (s *DoltStore) AddLabel(ctx context.Context, issueID, label, actor string) error {
+	_, err := s.db.ExecContext(ctx, `
+		INSERT IGNORE INTO labels (issue_id, label) VALUES (?, ?)
+	`, issueID, label)
+	if err != nil {
+		return fmt.Errorf("failed to add label: %w", err)
+	}
+	return nil
+}
+
+// RemoveLabel removes a label from an issue
+func (s *DoltStore) RemoveLabel(ctx context.Context, issueID, label, actor string) error {
+	_, err := s.db.ExecContext(ctx, `
+		DELETE FROM labels WHERE issue_id = ? AND label = ?
+	`, issueID, label)
+	if err != nil {
+		return fmt.Errorf("failed to remove label: %w", err)
+	}
+	return nil
+}
+
+// GetLabels retrieves all labels for an issue
+func (s *DoltStore) GetLabels(ctx context.Context, issueID string) ([]string, error) {
+	rows, err := s.db.QueryContext(ctx, `
+		SELECT label FROM labels WHERE issue_id = ? ORDER BY label
+	`, issueID)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get labels: %w", err)
+	}
+	defer rows.Close()
+
+	var labels []string
+	for rows.Next() {
+		var label string
+		if err := rows.Scan(&label); err != nil {
+			return nil, fmt.Errorf("failed to scan label: %w", err)
+		}
+		labels = append(labels, label)
+	}
+	return labels, rows.Err()
+}
+
+// GetLabelsForIssues retrieves labels for multiple issues
+func (s *DoltStore) GetLabelsForIssues(ctx context.Context, issueIDs []string) (map[string][]string, error) {
+	if len(issueIDs) == 0 {
+		return make(map[string][]string), nil
+	}
+
+	placeholders := make([]string, len(issueIDs))
+	args := make([]interface{}, len(issueIDs))
+	for i, id := range issueIDs {
+		placeholders[i] = "?"
+		args[i] = id
+	}
+
+	query := fmt.Sprintf(`
+		SELECT issue_id, label FROM labels
+		WHERE issue_id IN (%s)
+		ORDER BY issue_id, label
+	`, strings.Join(placeholders, ","))
+
+	rows, err := s.db.QueryContext(ctx, query, args...)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get labels for issues: %w", err)
+	}
+	defer rows.Close()
+
+	result := make(map[string][]string)
+	for rows.Next() {
+		var issueID, label string
+		if err := rows.Scan(&issueID, &label); err != nil {
+			return nil, fmt.Errorf("failed to scan label: %w", err)
+		}
+		result[issueID] = append(result[issueID], label)
+	}
+	return result, rows.Err()
+}
+
+// GetIssuesByLabel retrieves all issues with a specific label
+func (s *DoltStore) GetIssuesByLabel(ctx context.Context, label string) ([]*types.Issue, error) {
+	rows, err := s.db.QueryContext(ctx, `
+		SELECT i.id FROM issues i
+		JOIN labels l ON i.id = l.issue_id
+		WHERE l.label = ?
+		ORDER BY i.priority ASC, i.created_at DESC
+	`, label)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get issues by label: %w", err)
+	}
+	defer rows.Close()
+
+	var issues []*types.Issue
+	for rows.Next() {
+		var id string
+		if err := rows.Scan(&id); err != nil {
+			return nil, fmt.Errorf("failed to scan issue id: %w", err)
+		}
+		issue, err := s.GetIssue(ctx, id)
+		if err != nil {
+			return nil, err
+		}
+		if issue != nil {
+			issues = append(issues, issue)
+		}
+	}
+	return issues, rows.Err()
+}