chore(agents): add title to Starlark rules file

Add the missing H1 title to the Starlark rules file to ensure it conforms to the workspace rule formatting guidelines.

Author: rickeylev

.agents/rules/bzl.md

@@ -4,5 +4,7 @@ description: Starlark / Bazel .bzl file coding style rules
 globs: *.bzl
 ---
 
+# Starlark Rules
+
 * Use triple-quoted strings for multi-line rule doc args.
 * Don't use backslash line continuation in rule doc args.