Use the explicit merge-queue enqueue mutation

[philcunliffe]

Summary:

• add a deterministic neutral enqueue command
• guard the reviewed head before and during enqueue
• call GitHub enqueuePullRequest directly instead of falling back to repository auto-merge
• keep the reconcile skill as a small relay to the tested command

This follows the live HypAware queue result from hyparam/hypaware#965.

Verification:

• npm test (226 passing)
• npm run typecheck
• git diff --check