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/install.md

@@ -0,0 +1,31 @@
+---
+source_keys:
+  - context7-pre-commit-com
+  - pre-commit-com
+---
+
+# Installing hooks into `.git/hooks/`
+
+Reached from `SKILL.md`'s Route table when the user asks to install or set up hooks. Self-contained.
+
+Only run this flow when the user explicitly asks for it. Installing rewrites their clone's `.git/hooks/`; it is never a side effect of another request.
+
+## Gate — existing hook files
+
+Check `ls .git/hooks/` first. With hook files already there, `pre-commit install` does not refuse — it silently enters migration mode and runs both. Only `-f` replaces them, and `pre-commit uninstall` cannot restore whatever `-f` overwrote.
+
+So when hook files are present, put the choice to the user in these terms, including the irreversibility, and wait for an answer before passing `-f`:
+
+> "Existing hook files found in `.git/hooks/`. Plain `pre-commit install` runs both; `-f` overwrites them permanently and `pre-commit uninstall` cannot restore them. Plain install, or overwrite?"
+
+## Install
+
+```bash
+pre-commit install
+```
+
+Re-run with `-t` flags when `default_install_hook_types` changed, or when hooks in a non-default stage never fire — a hook whose stage was never installed cannot run:
+
+```bash
+pre-commit install -t pre-commit -t pre-push -t commit-msg
+```