ci: add Docker image pull validation job (Phase 2)

- Add image-pull job to .gitea/workflows/ci.yml that detects image
  changes in *.nomad.hcl files on PRs and pulls each changed image
- Remove act-runner labels config — default runner already uses
  docker.gitea.com/runner-images:ubuntu-latest which has docker CLI
- Remove CONFIG_FILE env var from act-runner (no longer needed)
- Mark Phase 2 complete in cicd-plan.md

cicd-plan.md

@@ -296,7 +296,7 @@ exit 1
 - [x] **Phase 1a**: Create `act-runner.nomad.hcl` + Terraform wrapper, register runner token in Gitea, get a hello-world workflow green
 - [x] **Phase 1b**: Add `terraform fmt` + `terraform validate -backend=false` workflow — no secrets needed
 - [x] **Phase 1c**: Add Nomad validate step — add `NOMAD_ADDR` + read-only `NOMAD_TOKEN` to Gitea secrets
-- [ ] **Phase 2**: Add image pull validation step to the workflow
+- [x] **Phase 2**: Add image pull validation step to the workflow
 - [ ] **Phase 3a**: Add `update` stanzas to ntfy and glance (simplest, no volume conflict)
 - [ ] **Phase 3b**: Add rolling `update` stanzas to remaining service jobs (jellyfin, sonarr, etc.)
 - [ ] **Phase 3c**: Add health checks to openreader and unifi before adding update stanzas