docs(agents): never commit review artifacts to a contributor's fork (#286)

The artifact rule said "PR review -> on the PR branch" without qualifying who owns it, so reviewing a community PR meant pushing to the contributor's branch. That happened on #274 and #277 this week; both were reverted.

maintainerCanModify makes the push possible; it is not permission. It also backfires mechanically: this repo dismisses stale reviews on push, so the review-doc commit dismisses the approval it documents and staleness-flags the approval label against the new head.

Fork PRs now have no committed artifact — the formal review body is the artifact.

Closes #286

Author: vsits-proxy-builder[bot]

AGENTS.md

@@ -19,12 +19,33 @@ cache-fix-specific context.
 This repo uses **`docs/code-reviews/`** (not `docs/reviews/` — older
 convention retained for continuity). Commit review documents there:
 
-- **PR review** → on the PR branch as
+- **PR review, PR branch owned by this org** → on the PR branch as
   `docs/code-reviews/pr-<N>-round-<R>-codex.md`
+- **PR review, PR from a contributor's fork** → **do not push anything to
+  the fork.** See below.
 - **Issue / directive review** → on a review branch (or the directive's
   branch if one exists) as
   `docs/code-reviews/issue-<N>-round-<R>-codex.md`
 
+### Never write to a contributor's fork
+
+When the PR head is a fork you do not own, the review artifact does **not**
+get committed anywhere. Post the review body in the formal `gh pr review`
+and stop there — that is the artifact.
+
+`maintainerCanModify: true` makes pushing mechanically possible on most
+community PRs. It is not permission. A contributor's branch is theirs; a
+push you make to it rewrites history they may have local work on, and it
+puts our internal review notes into their PR's file diff where they do not
+belong.
+
+There is also a mechanical cost: this repo dismisses stale reviews on push,
+so a review-doc commit **dismisses the very approval it documents**, and
+any `approved-by-*` label instantly goes stale against the new head.
+
+Check before writing: if `gh pr view <N> --json headRepositoryOwner` is not
+this org, no commit.
+
 ## What cache-fix Is
 
 The proxy that replaces the Node.js `--import` preload interceptor