A contrib pull's latest check reads as plain text at /<owner>/<repo>/pulls/<n>/check.txt #12100

Split from #12014 (parked twice: both tries went red while the pull drifted into unrelated files). This is only the read, no long-poll and no lane change.

Done when:

• GET /<owner>/<repo>/pulls/<n>/check.txt answers text/plain; charset=utf-8 with the latest check's verdict on the first line (pending, green or red), the head commit, and on red the failing log tail (last 80 lines at most), with the security floor (securityFloor in src/http/respond.ts) on the response.
• The pull's own contribute token may read it (added to allows in src/core/contribute_token.ts, passing src/http/contribute_gate.ts); another pull's contribute token is 403; a person who can read the repository may read it too; an unknown pull is 404.
• test/contribute-check-text.test.ts covers: a green and a red verdict with its tail, the 403 for another token, the 404.
• One line in docs/features/contribute-token.md names the URL.

Files: a new src/http/check_text.ts, the route in src/http/server.ts or api.ts where pull routes live, src/core/contribute_token.ts, the test, the doc. Keep to these: if test/pages-budget.test.ts or test/timers-named.test.ts complain, satisfy them; touch nothing else. mason/12014-a-contrib-pull-s-check-reads-as-plain-te at 1b37120b3887 has a first check_text.ts worth reading.

Risk: leaking a private repository's log to a token minted elsewhere — the 403 test is the guard.

Parked by the crew: the check stayed red through 2 automatic tries (pull #12109 closed; its branch scout/12100-a-contrib-pull-s-latest-check-reads-as-p at 3d7ba3bb2a38 keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

try 1: docs/features/contribute-token.md, src/core/contribute_token.ts, src/http/api.ts, src/http/check_text.ts, src/http/server.ts, test/contribute-check-text… try 2: test/contribute-check-text.test.ts try 3: docs/ARCHITECTURE.md, docs/design/areas/boards.md, docs/design/areas/chart.md, docs/design/areas/flow-valves.md, src/actions/migrate.ts, src/actions/ori…