Add slim Phase 1 task 1.11 (Dockerfile + Gitea workflow) for pilot deploy

- Multi-stage Dockerfile (Node 22 alpine, BuildKit cache, non-root user).
  HEALTHCHECK and metrics port (9090) deferred until task 1.10 ships;
  comments document the resume.
- .gitea/workflows/build.yml — single build job following the pattern
  of other TRM repos (no services/container, ubuntu-latest direct).
  Tests + typecheck + lint inline; image tagged :main.
- compose.dev.yaml — local-build variant for verifying Dockerfile
  changes pre-push. Production deploy lives in the sibling deploy/ repo.
- .env.example documenting all runtime env vars.
- README updated to point at deploy/ for production and explain CI.
- Task 1.11 marked done (slim variant) in ROADMAP and task file.

.dockerignore

@@ -3,8 +3,10 @@ dist/
 coverage/
 .env
 .env.local
+.env.example
 *.log
 .git/
+.gitea/
 .planning/
 test/
 *.md