[TheGreatAxios]

Summary

Whether Triage may merge a pull request is now decided by one function, mergeGate in triage-contracts. It returns either allowed or a typed refusal with a cause (changes requested, review threads, or CI) and a reason.

• The evaluation (heldApprovals) and both executors (GitHub do.ts, Bitbucket action.ts) call it. Their own copies of the CI, changes-requested and thread checks are gone.
• mergeableCi is replaced by the gate. ciRefusal and the head CI type move from github-reads to triage-contracts.
• Evaluation now also holds a merge while another reviewer requests changes, using the existing changesRequested fact. The merge is skipped and shown as waiting, the same way an open review thread holds it.
• Bitbucket's mergeVeto still runs after the gate.
• The portal's Merge button is unchanged here. It moves onto the gate in a follow-up PR.

Verification

• bun run check and bun run lint pass.
• bun --conditions=intx-src test ./packages ./tests/model (local Postgres): 764 pass, 0 fail.
• bun --conditions=intx-src test --parallel=2 tests/chain: 13 pass, 0 fail.
• New model check, "a merge waits while a reviewer requests changes": there is no merge write, and the comment says "Changes are requested by maya."

Refs CL-12170

[linear-code]

CL-12170

[corbits-triage]

Merged before Triage finished on f48cf6d.