Smoke-5 surfaced that be-dev-1 had no gateway-native way to fix a
PR's title/body or request a reviewer after open_pr; `gh pr edit`
is bash-shimmed and the dev correctly escalated rather than bypass
the guard. This adds the GitService primitive: PATCH /pulls/{n}
for title/body and POST /pulls/{n}/requested_reviewers for the
reviewer list, with NotFound + 422 mapped to typed GitError. The
PRUpdateRequest schema enforces 'at least one field' via a
model_validator so the route returns 422 before reaching the verb.
