feat: add golang-database skill with best practices and reference documentation

.openclaw/workspace/skills/golang-database/references/transactions.md

@@ -0,0 +1,49 @@
+# Transactions, Isolation Levels, and Locking
+
+## Basic transaction pattern
+
+```go
+tx, err := db.BeginTxx(ctx, nil) // default isolation (READ COMMITTED)
+if err != nil {
+    return fmt.Errorf("beginning transaction: %w", err)
+}
+defer tx.Rollback() // no-op if already committed
+
+// ... execute queries using tx ...
+
+if err := tx.Commit(); err != nil {
+    return fmt.Errorf("committing transaction: %w", err)
+}
+```
+
+## Custom isolation level
+
+```go
+tx, err := db.BeginTxx(ctx, &sql.TxOptions{
+    Isolation: sql.LevelSerializable, // strongest guarantee
+})
+```
+
+| Level | Use when |
+| --- | --- |
+| `LevelReadCommitted` | Default — good for most operations |
+| `LevelRepeatableRead` | Need consistent reads within a transaction |
+| `LevelSerializable` | Financial operations, inventory, anything with strict consistency |
+
+## SELECT FOR UPDATE — prevent race conditions
+
+```go
+var balance int
+err := tx.GetContext(ctx, &balance, "SELECT balance FROM accounts WHERE id = $1 FOR UPDATE", accountID)
+// Row is locked until tx.Commit() or tx.Rollback()
+```
+
+Use `FOR UPDATE` when you read a value, compute something from it, and then write it back. Without the lock, concurrent transactions can read stale data.
+
+## Locking variants
+
+| Clause | Effect |
+| --- | --- |
+| `FOR UPDATE` | Locks rows for write — other transactions block on same rows |
+| `FOR UPDATE NOWAIT` | Same, but fails immediately instead of waiting |
+| `FOR SHARE` | Locks rows for read — prevents writes but allows other reads |