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

2-nomad-config/act-runner.nomad.hcl

@@ -29,7 +29,6 @@ job "act-runner" {
 
       env = {
         GITEA_INSTANCE_URL = "https://gitea-1ef0bea6b75a4fd3e9393a9f7f7e4b02.othrayte.one"
-        CONFIG_FILE        = "/secrets/runner-config.yml"
       }
 
       # Required SOPS key:
@@ -43,19 +42,6 @@ EOF
         env         = true
       }
 
-      # Limit which images/labels the runner will accept so it doesn't pick up
-      # unrelated workloads if more runners are added later.
-      template {
-        data        = <<EOF
-runner:
-  labels:
-    - "ubuntu-latest:docker://node:20-bookworm"
-    - "ubuntu-22.04:docker://node:20-bookworm"
-    - "ubuntu-24.04:docker://node:20-bookworm"
-EOF
-        destination = "secrets/runner-config.yml"
-      }
-
       resources {
         cpu        = 200
         memory     = 256