fix(sqlite): add retry logic to transaction entry points (GH#1272)

Update withTx to use BEGIN IMMEDIATE with exponential backoff retry
on SQLITE_BUSY errors. This prevents "database is locked" failures
during concurrent operations (daemon + CLI, multi-agent workflows).

Changes:
- withTx now uses beginImmediateWithRetry (same pattern as RunInTransaction)
- Add dbExecutor interface for helper functions that work with both
  *sql.Tx and *sql.Conn
- Update all withTx callers to use *sql.Conn
- Refactor DeleteIssue to use withTx (fixes the specific error in auto-import)
- Update markIssuesDirtyTx to accept dbExecutor interface

Affected paths:
- MarkIssuesDirty, ClearDirtyIssuesByID (dirty.go)
- AddDependency, RemoveDependency (dependencies.go)
- executeLabelOperation (labels.go)
- AddComment (events.go)
- ApplyCompaction (compact.go)
- DeleteIssue (queries.go)

Note: Some direct BeginTx calls in queries.go (CloseIssue, UpdateIssue,
ReopenIssue, DeleteIssues) still use the old pattern and could be
refactored in a follow-up.

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

internal/storage/sqlite/labels.go

@@ -19,8 +19,8 @@ func (s *SQLiteStorage) executeLabelOperation(
 	eventComment string,
 	operationError string,
 ) error {
-	return s.withTx(ctx, func(tx *sql.Tx) error {
-		result, err := tx.ExecContext(ctx, labelSQL, labelSQLArgs...)
+	return s.withTx(ctx, func(conn *sql.Conn) error {
+		result, err := conn.ExecContext(ctx, labelSQL, labelSQLArgs...)
 		if err != nil {
 			return fmt.Errorf("%s: %w", operationError, err)
 		}
@@ -34,7 +34,7 @@ func (s *SQLiteStorage) executeLabelOperation(
 			return nil
 		}
 
-		_, err = tx.ExecContext(ctx, `
+		_, err = conn.ExecContext(ctx, `
 			INSERT INTO events (issue_id, event_type, actor, comment)
 			VALUES (?, ?, ?, ?)
 		`, issueID, eventType, actor, eventComment)
@@ -43,7 +43,7 @@ func (s *SQLiteStorage) executeLabelOperation(
 		}
 
 		// Mark issue as dirty for incremental export
-		_, err = tx.ExecContext(ctx, `
+		_, err = conn.ExecContext(ctx, `
 			INSERT INTO dirty_issues (issue_id, marked_at)
 			VALUES (?, ?)
 			ON CONFLICT (issue_id) DO UPDATE SET marked_at = excluded.marked_at