Upgrade forgejo-runner 12.7.0 → 12.7.3, add service card

Patch upgrade picks up idempotent FetchTask API, offline registration
fix, cloudflare/circl security dep update, and custom gRPC user-agent.
No config defaults changed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

argocd/manifests/forgejo-runner/config.yaml

@@ -1,4 +1,4 @@
-# Reviewed against v12.7.0 defaults (2026-02-22)
+# Reviewed against v12.7.3 defaults (2026-03-30)
 log:
   level: info
 

argocd/manifests/forgejo-runner/kustomization.yaml

@@ -10,7 +10,7 @@ resources:
 
 images:
   - name: code.forgejo.org/forgejo/runner
-    newTag: "12.7.0"
+    newTag: "12.7.3"
   - name: docker
     newTag: 27-dind