docs(stack): bind vendored stack-update skill provenance

Author: erinepshovel-code

.agents/skills/README.md

@@ -0,0 +1,12 @@
+# Vendored agent skills
+
+This directory contains repo-local consumed copies of canonical skills from
+`The-Interdependency/skill-lib`.
+
+Installed subset:
+
+- `stack-update` — source `The-Interdependency/skill-lib@a7b95f891d2e88c62e61524dc52a2e1577c9983d`, path `stack-update/SKILL.md`, source blob `a1e914893fa047e28d039050395937a1cf6e0138`.
+
+Canonical doctrine remains in `skill-lib`; vendoring does not transfer authority.
+Structural stack changes must follow `.agents/skills/stack-update/SKILL.md` and pass
+`python tools/check_stack_consistency.py` before merge.