feat: add delete, context, and tx helpers

Add Delete/DeleteAffected, context-aware CRUD helpers, WithTx, ContainsCol,
and queryReturning deduplication for returning statements.

Co-authored-by: Cursor <cursoragent@cursor.com>

tx.go

@@ -0,0 +1,26 @@
+package dbx
+
+import (
+	"context"
+	"database/sql"
+)
+
+// WithTx begins a transaction on db, runs fn, and commits on success.
+// The transaction is rolled back if fn returns an error or commit fails.
+func WithTx(ctx context.Context, db QueryExecTx, fn func(tx *sql.Tx) error) error {
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+
+	tx, err := db.Begin()
+	if err != nil {
+		return err
+	}
+
+	if err := fn(tx); err != nil {
+		_ = tx.Rollback()
+		return err
+	}
+
+	return tx.Commit()
+}