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/util_test.go

@@ -104,8 +104,8 @@ func TestExecInTransaction(t *testing.T) {
 	defer store.Close()
 
 	t.Run("successful transaction", func(t *testing.T) {
-		err := store.ExecInTransaction(ctx, func(tx *sql.Tx) error {
-			_, err := tx.ExecContext(ctx, "INSERT INTO config (key, value) VALUES (?, ?)", "test_key", "test_value")
+		err := store.ExecInTransaction(ctx, func(conn *sql.Conn) error {
+			_, err := conn.ExecContext(ctx, "INSERT INTO config (key, value) VALUES (?, ?)", "test_key", "test_value")
 			return err
 		})
 		if err != nil {
@@ -125,8 +125,8 @@ func TestExecInTransaction(t *testing.T) {
 
 	t.Run("failed transaction rolls back", func(t *testing.T) {
 		expectedErr := errors.New("intentional error")
-		err := store.ExecInTransaction(ctx, func(tx *sql.Tx) error {
-			_, err := tx.ExecContext(ctx, "INSERT INTO config (key, value) VALUES (?, ?)", "rollback_key", "rollback_value")
+		err := store.ExecInTransaction(ctx, func(conn *sql.Conn) error {
+			_, err := conn.ExecContext(ctx, "INSERT INTO config (key, value) VALUES (?, ?)", "rollback_key", "rollback_value")
 			if err != nil {
 				return err
 			}