check.txt holds with ?wait= until a pending check's verdict changes #12101

Split from #12014. The long-poll on the plain-text check.

Done when:

• GET …/pulls/<n>/check.txt?wait=S with S in 1..120 holds the request until the latest check's verdict differs from the one it began with, or S seconds pass, then answers the same text as without wait; S outside 1..120 or not an integer is a 400 naming the range.
• The hold listens to the check's existing event (no polling the database in a loop) and releases its listener and named timer on every exit, including the client going away.
• A test starts a pending check, long-polls it, finishes the check, and sees the new verdict return before the wait ends; a second test sees a wait end at its limit with pending.
• The doc line from the first issue names ?wait=.

Files: src/http/check_text.ts, test/contribute-check-text.test.ts, docs/features/contribute-token.md. Nothing else.

Risk: a held request per token is a cost; the contribute gate's per-minute limit still applies and the count of held requests per token is capped at 2 (third is 429).