refactor(pc-run): retrofit to the ADR-0020 context contract

Description 477 -> 211 chars, body 736 -> 367 words. Install, autoupdate,
and clean become flow files behind the Route table.

The audit found no route for 'hooks aren't running': the only pointer to
failure-patterns.md sat inside the failure path, but when hooks never fire
the manual run succeeds, so the request fell to the ambiguity default and
was masked. Restores the disclosure that install -f is not reversible by
uninstall.

plugins/git/skills/pc-run/references/autoupdate.md

@@ -0,0 +1,17 @@
+---
+source_keys:
+  - context7-pre-commit-com
+  - pre-commit-com
+---
+
+# Bumping hook revs with `autoupdate`
+
+Reached from `SKILL.md`'s Route table when the user asks to update hook versions or bump revs. Self-contained.
+
+```bash
+pre-commit autoupdate
+```
+
+This rewrites `.pre-commit-config.yaml` in place, so re-read the file afterwards and report which `rev` values changed. It is the one operation in this skill that writes that file, and the exception is deliberate: the rewrite is pre-commit's own, resolved against the hook repos, not a hand edit — which is why `pc-author` hands rev bumps here rather than making them itself.
+
+Add `--freeze` when the user wants exact SHAs pinned for reproducibility.