feat(ci): add GC CronJob for runner cleanup, expand reg PVCs to 20Gi

- Add gc-cronjob.yaml: daily prune of DinD Docker images/volumes/build-cache
  and actcache across all forgejo-runner pods. Keeps :latest tagged images,
  deletes non-latest older than 72h.
- Expand runner reg PVCs from 1Gi to 20Gi (all three runners) to prevent
  action tool cache from filling disk.
- Rust runner gets explicit 20Gi persistence override.
- GC only renders from golang (default) values to avoid duplicate resources.

k8s/infra/forgejo-runner/values-rust.yaml

@@ -8,3 +8,12 @@
 runner:
   name: rust-runner
   labels: "rust:docker://rust:1.83-bookworm"
+
+persistence:
+  reg:
+    storageClass: longhorn
+    size: 20Gi
+
+# GC CronJob renders only from the default (golang) values to avoid duplicates
+gc:
+  enabled: false