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>

doc.go

@@ -7,14 +7,13 @@
 //
 // # Query and mutation helpers
 //
-// [Fetch], [FetchOne], [Insert], and [Update] wrap Jet [Statement] execution
-// with consistent error semantics. Must* variants return a caller-provided error
-// when no rows are found.
+// [Fetch], [FetchOne], [Insert], [Update], [Delete], and their Must* and Context
+// variants wrap Jet [Statement] execution with consistent error semantics.
 //
 // # Jet column utilities
 //
 // Dialect-neutral type aliases ([Column], [ColumnList]) and helpers for column
-// lists ([NormalCols]) and expression building ([ExprValues]).
+// lists ([NormalCols], [ContainsCol]) and expression building ([ExprValues]).
 //
 // Partial-update helpers ([ApplyPtr], [ApplyVal]) track changed fields for
 // repository patch logic.