feat(github-app): wire check-run annotations output

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Parent roadmap: #525
Parent epic: #537

Goal

Wire output.annotations[] into createOrUpdateNamedCheckRun for the Gittensory Context check (Gate stays a clean binary).

Reuse

src/github/app.ts:165.

Acceptance

Context check publishes annotations on changed lines; Gate unchanged.

Testing

npm run test:ci, 97%+ coverage. Public/private output boundary preserved; deterministic where applicable.