fix: Enable Eve examples pull requests (#13708)
## Summary
- Run examples maintenance against a fresh sandbox checkout instead of
the Vercel function filesystem.
- Let the weekly schedule update, validate, and publish example changes
as a draft pull request.
- Exchange Vercel OIDC for a short-lived, repository-scoped GitHub token
through an environment-configured token broker.
- Make pull request publication idempotent and constrain automated
branches, files, and metadata.
## Deployment
Configure `GITHUB_TOKEN_EXCHANGE_URL` in the production agent project
before triggering the schedule. The broker-specific endpoint remains
outside this public repository.
## Verification
- `pnpm --dir apps/agents exec tsgo --noEmit`
- `pnpm --dir apps/agents build`
- Pre-push formatting and TOML checks
- `eve info` reports zero diagnostics
