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/.apm/skills/pc-run/references/clean.md

@@ -0,0 +1,21 @@
+---
+source_keys:
+  - context7-pre-commit-com
+  - pre-commit-com
+---
+
+# Wiping the pre-commit cache
+
+Reached from `SKILL.md`'s Route table when the user asks to clean the cache or rebuild environments from scratch. Self-contained.
+
+## Gate — confirm first
+
+`pre-commit clean` wipes the whole cache at `~/.cache/pre-commit`, forcing every hook environment to be re-downloaded on the next run. Require explicit confirmation before executing it:
+
+> "This will wipe the entire pre-commit cache. All hook environments will be re-downloaded on next run. Proceed?"
+
+```bash
+pre-commit clean
+```
+
+Prefer `pre-commit gc` when the goal is only to reclaim disk — it drops unused environments and leaves the ones in use intact, so it needs no confirmation.