ci: fix runner labels + CoreDNS rewrite + cleanup

- Runners use public images (code.forgejo.org/forgejo/runner:6)
- Labels pull from Docker Hub: golang:1.26, node:22, rust:1-bookworm
- Add CoreDNS api.riotpiao.com rewrite
- Fix runner re-registration to keep labels in sync
- Add unified CI pattern docs to CLAUDE.example.md
- Remove dead .forgejo/ workflow dir (Forgejo uses .gitea/)

k8s/infra/forgejo-runner/templates/configmap.yaml

@@ -36,3 +36,4 @@ data:
       valid_volumes:
         - /docker-certs/client
       network: host
+      docker_host: automount

k8s/infra/forgejo-runner/templates/deployment.yaml

@@ -34,7 +34,11 @@ spec:
           command: ["sh", "-c"]
           args:
             - |
-              test -f /data/.runner || forgejo-runner register --no-interactive \
+              # Always re-register to keep labels in sync with values.yaml.
+              # Without this, changing a runner label requires manually deleting
+              # the PVC or .runner file — not GitOps-friendly.
+              rm -f /data/.runner
+              forgejo-runner register --no-interactive \
                 --instance {{ .Values.runner.forgejoUrl }} \
                 --token $(RUNNER_TOKEN) \
                 --name {{ .Values.runner.name }} \
@@ -70,6 +74,8 @@ spec:
               mountPath: /data
             - name: docker-certs
               mountPath: /docker-certs
+            - name: docker-sock
+              mountPath: /run
             - name: homelab-ca
               mountPath: /etc/ssl/certs/homelab-ca.pem
               subPath: ca.crt
@@ -89,6 +95,8 @@ spec:
           volumeMounts:
             - name: docker-certs
               mountPath: /docker-certs
+            - name: docker-sock
+              mountPath: /run
             - name: dind-storage
               mountPath: /var/lib/docker
             - name: homelab-ca
@@ -113,6 +121,8 @@ spec:
             claimName: {{ .Release.Name }}-dind
         - name: docker-certs
           emptyDir: {}    # DinD regenerates mTLS certs on each start
+        - name: docker-sock
+          emptyDir: {}    # Shared docker socket between dind and runner
         - name: homelab-ca
           # homelab-ca is a ConfigMap (public CA trust bundle), not a Secret.
           # The volumeMounts use subPath: ca.crt to project the single cert file.