refactor: standardize workflow name and move env to workflow level

- Change name to 'CI' (consistent with template)
- Move REGISTRY, IMAGE to workflow-level env (shared by both jobs)
- Compact 'on:' branches syntax
- Maintain all service-specific features (Delete old latest, build-arg)

No functional change, pure standardization to match template pattern.

.gitea/workflows/build-push.yml

@@ -1,12 +1,14 @@
-name: Build & Push Portfolio Image
+name: CI
 
 on:
   push:
-    branches:
+    branches: [main]
-      - main
   pull_request:
-    branches:
+    branches: [main]
-      - main
+
+env:
+  REGISTRY: forgejo.riotpiao.com
+  IMAGE: forgejo.riotpiao.com/rock/portfolio
 
 jobs:
   test:
@@ -30,9 +32,6 @@ jobs:
     needs: test
     if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     runs-on: node
-    env:
-      REGISTRY: forgejo.riotpiao.com
-      IMAGE: forgejo.riotpiao.com/rock/portfolio
     steps:
       - name: Install Node.js and Docker
         run: |