Builders push migrations without their pin and sink merge batches: the crew's pre-push runs pin-migrations automatically when a migration changes #6938

Builders keep landing migrations without their pin, and each one sinks a merge batch. Tonight a migration on main is never edited (#783) > every migration is pinned by its own pull: run bun scripts/pin-migrations.ts turned these red: #6647 at 06:30Z (sinking 4 stacked prefixes), the batch of 8 at 09:38Z, and another at 10:09Z (cutting off 14 more pulls). Main passes the test, so it's always a pull that added a migration and didn't run the script.

Done when:

1. The crew's pre-push step runs bun scripts/pin-migrations.ts whenever the diff adds or changes a file under src/core/migrations/, and commits the pin. Do it in the builder's harness (the same place it runs tsc before pushing), not in a prompt the agent may skip.
2. The pull check names the fix in its summary when only this test fails ("this pull adds migration X without its pin: run …"), so a hand-back is one step.
3. Test: a fake builder run that adds a migration ends with the pin committed; a pull without it gets the named summary.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.