docs: add gh-aw guidance for agents

Author: mrjf

AGENTS.md

@@ -0,0 +1,12 @@
+# AGENTS.md
+
+## Agentic Workflow Sources
+
+When changing GitHub Agentic Workflow source files under
+`.github/workflows/`, run `gh aw compile` before committing and
+include the regenerated `.lock.yml` files in the same change.
+
+This applies to workflow markdown files such as `.github/workflows/*.md`
+and shared workflow markdown such as `.github/workflows/shared/*.md`.
+The `gh-aw lock check` CI job runs the same compile step and fails if
+the generated workflow files are stale.