refactor(ci): review pull requests with one senior-engineer role

The review job ran the official code-review plugin, which fans a pull
request out to five Sonnet reviewers plus a Haiku scorer per finding and
drops everything scored under 80, and the briefing file spent most of its
lines overriding that plugin. Both are gone: the job hands one Senior
Software Engineer prompt to the action inline, the way the issue analyst
does, and denies the Agent tool so the single role is mechanical rather
than a request.

REVIEW.md moves from the emoji markers to CRITICAL/HIGH/MEDIUM/LOW with a
pre-existing qualifier. The uncapped rule is scoped to findings the pull
request introduced or worsened so it cannot collide with the cap of three
pre-existing ones. "A finding is a report, not a patch" stays as it was.

Workflow housekeeping: GH_TOKEN, REPO and PR live in the job env instead
of six step copies; the skip gate is per pull request, so a head pushed
after the automatic review is reviewed only on @claude review; the comment
counters sum gh's per-page jq output, which read "0\n0" as a review on a
pull request with more than 100 comments; --max-turns rises to 300 because
every read now costs the single agent a turn instead of a subagent.

.github/claude/repo-context.md

@@ -3,9 +3,8 @@
 Briefing for the issue analyst in `.github/workflows/claude-issue-analyst.yml`.
 It exists so these facts live in ONE place next to the code instead of being
 restated in the prompt, where they went stale silently. (Pull-request review is
-separate: the code-review skill in `.github/workflows/claude-pr-review.yml` is
-briefed with `CLAUDE.md`, `REVIEW.md` and `.github/claude/review-job.md`, not
-this.)
+separate: the reviewer in `.github/workflows/claude-pr-review.yml` is briefed by
+its own prompt, `CLAUDE.md` and `REVIEW.md`, not this.)
 
 `CLAUDE.md`, `frontend/CLAUDE.md` and `docs/architecture.md` outrank this file.
 Where they disagree with it, they win and this file is the thing to fix.