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>

jet_columns.go

@@ -1,6 +1,15 @@
 package dbx
 
-import "github.com/go-jet/jet/v2/mysql"
+import (
+	"slices"
+
+	"github.com/go-jet/jet/v2/mysql"
+)
+
+// ContainsCol reports whether cols contains col.
+func ContainsCol(cols ColumnList, col Column) bool {
+	return slices.Contains(cols, col)
+}
 
 // NormalCols processes a list of columns and strips out any that implement any of
 // ColumnTimestamp, ColumnTime, or ColumnDate.