Add GET /trigger/eval route to eval-webhook with HMAC-signed URLs.
The showcase_eval_check.yml workflow now posts a bot comment with a
clickable "Run Evaluation" link. Clicking triggers the eval and
redirects back to the PR. The link is signed so it can't be forged.
Hono web server that receives check_run.requested_action webhooks from
GitHub, authenticates as the devops bot, updates the Check Run to
in_progress, and dispatches showcase_eval.yml via workflow_dispatch.
Includes GHCR build workflow and pnpm workspace registration.