Cut a wave: an admin opens a draft waves/<next>.md as a pull listing every pull merged since the last wave #1508

Part of #1125, split smaller.

Done when:

• [ ] Settings shows Cut a wave to repository admins only. It is a same-origin POST. It opens a pull that adds waves/<next>.md, written with writeWave. The full list comes from the pulls merged since the last wave's date. The headline and highlights are left as clearly marked placeholders for a person or the crew. No model call.
• [ ] If an earlier wave does not parse, the cut stops and names that file. Nothing is written.
• [ ] A cut with no merged pulls since the last wave is refused with a plain reason.
• [ ] Tests: the list matches the merged pulls; a non-admin is refused; a cross-origin POST is refused; a malformed earlier wave stops the cut and is named. Any new test file goes in GATE in scripts/gate.ts.

Files: the Settings action, src/story/ (a small cut module), test/.

Parked by the crew: the check failed ("asks that expire, hold and collapse > two green pulls in one repository are one row, and every surface counts it once") and the decider saw no point in another try (pull #1531 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("the asks page > when nothing asks, it says All fine, and so does the header") and the decider saw no point in another try (pull #1558 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed ("asks that expire, hold and collapse > two green pulls in one repository are one row, and every surface counts it once") and the decider saw no point in another try (pull #1531 closed)
2. the check failed ("the asks page > when nothing asks, it says All fine, and so does the header") and the decider saw no point in another try (pull #1558 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

replaced by #1627. #1084 now waits on it. Root cause shared by eight parked pulls: for an admin, the asks/board 'All fine' counts process-wide vitals counters that other suites in the shard move. That is fixed first in #1614, and every replacement waits on it.