fix(pc-run): rebind the clean gate and fixer-hook rule to every branch

SKILL.md tells a dispatching agent to read the matching reference file
"and no other". The retrofit moved the `pre-commit clean` confirmation
gate out of the always-loaded body into references/clean.md, but
references/failure-patterns.md — loaded by the diagnosis route, not the
clean route — prescribes `pre-commit clean` with no gate at all. So "why
is this hook failing" could wipe the machine-wide cache at
~/.cache/pre-commit for every repo without asking.

The gate returns to the body, where every branch loads it, and is
restated at the point of use in failure-patterns.md. It is its own
section rather than a Gotchas bullet because folding it in pushed the
Gotchas ratio to 41%, whose only suggested remedy is moving it back to
references/ — the move that caused this.

The fixer-hook rule had the same shape: reachable only behind "if the
cause is not obvious from the output", which is false precisely when
pre-commit prints `- files were modified by this hook`. The fix
(`git add -u && git commit`) and the prohibition on `pre-commit install -f`
are now unconditional, and the two weakened pointers that stranded them
are restored.

Found by an independent review of this branch.

Refs #99

plugins/git/skills/pc-run/references/failure-patterns.md

@@ -89,13 +89,14 @@ Fix: The user (or `pc-author`) must add `args: [--autofix]` to the hook override
 
 Cause: A hook's cached environment is corrupted or out of date.
 
-Fix:
+Fix: `pre-commit clean` is gated. It wipes the machine-wide cache at `~/.cache/pre-commit`, shared by every repo on the box, so get explicit confirmation before running it — "This will wipe the entire pre-commit cache. All hook environments will be re-downloaded on next run. Proceed?"
+
 ```bash
-pre-commit clean          # wipe all environments
+pre-commit clean          # gated — confirm with the user first
 pre-commit install-hooks  # rebuild everything
 ```
 
-Or less destructively:
+Or less destructively, needing no confirmation:
 ```bash
 pre-commit gc             # remove only unused environments
 ```