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

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

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+# Struct Scanning and NULLable Columns
+
+## Struct Scanning with sqlx
+
+Tag struct fields with `db:"column_name"` for sqlx:
+
+```go
+type User struct {
+    ID        int64      `db:"id"`
+    Name      string     `db:"name"`
+    Email     string     `db:"email"`
+    DeletedAt *time.Time `db:"deleted_at"` // NULLable
+}
+
+// Single row
+var user User
+err := db.GetContext(ctx, &user, "SELECT id, name, email, deleted_at FROM users WHERE id = $1", id)
+
+// Multiple rows
+var users []User
+err := db.SelectContext(ctx, &users, "SELECT id, name, email, deleted_at FROM users WHERE active = true")
+```
+
+## Struct Scanning with pgx
+
+With pgx (v5+), use `pgx.CollectRows` for automatic struct mapping:
+
+```go
+rows, err := pool.Query(ctx, "SELECT id, name, email FROM users WHERE active = true")
+if err != nil {
+    return fmt.Errorf("querying users: %w", err)
+}
+users, err := pgx.CollectRows(rows, pgx.RowToStructByName[User])
+```
+
+## JSON Marshaling
+
+Struct tags for both database and JSON work together. Pointer fields marshal to `null` in JSON when NULL in the database:
+
+```go
+type User struct {
+    ID        int64      `db:"id"         json:"id"`
+    Name      string     `db:"name"       json:"name"`
+    Email     string     `db:"email"      json:"email"`
+    Bio       *string    `db:"bio"        json:"bio,omitempty"` // NULL → omitted in JSON
+    DeletedAt *time.Time `db:"deleted_at" json:"deleted_at"`    // NULL → null in JSON
+}
+```
+
+## NULLable Columns
+
+Three approaches, from most to least recommended:
+
+**1. Pointer fields (recommended)** — clean, works with JSON marshaling:
+
+```go
+type User struct {
+    ID        int64      `db:"id"    json:"id"`
+    Name      string     `db:"name"  json:"name"`
+    DeletedAt *time.Time `db:"deleted_at" json:"deleted_at"` // nil when NULL
+}
+// Check: if user.DeletedAt != nil { ... }
+```
+
+**2. `sql.NullXxx` types** or `sql.Null[T]` generic — explicit but verbose, requires custom JSON marshaling:
+
+```go
+type User struct {
+    ID        int64          `db:"id"`
+    Bio       sql.NullString `db:"bio"`
+}
+// Check: if user.Bio.Valid { use(user.Bio.String) }
+```
+
+**3. `COALESCE` in SQL** — moves NULL handling to the query:
+
+```sql
+SELECT id, COALESCE(bio, '') AS bio FROM users WHERE id = $1
+```