ci: remove custom runner image approach (use public Docker Hub images instead)

Custom runner images were never needed. Forgejo runners pull label images
(golang:1.26-bookworm, node:22-bookworm, rust:1-bookworm) from Docker Hub
at job time. Node.js/docker are installed in workflow steps.

Removes dead-end Dockerfiles and build workflow that couldn't push through
Cloudflare's 100MB payload limit.

CLAUDE.example.md

@@ -208,3 +208,95 @@ versions without warning in your own values file.
 Grouping by layer (rather than by day or by "misc fixes") makes it much
 easier to `git log --oneline -- <path>` your way back to *why* a given
 piece of config looks the way it does, months later.
+
+## Unified Forgejo CI Workflow Pattern (Enforced 2026-09-07+)
+
+All repositories MUST follow this exact structure. No variations.
+
+```yaml
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  REGISTRY: <your-registry-hostname>
+  IMAGE: <registry>/<org>/<service-name>
+
+jobs:
+  test:
+    name: Test
+    runs-on: [golang|node|rust]
+    steps:
+      - name: Install Node.js for actions runtime
+        run: apt-get update && apt-get install -y nodejs
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      # Language-specific tests here (no docker, no registry)
+      # - name: Run tests
+      #   run: npm test -- --run || true
+
+  build-push:
+    name: Build & Push Image
+    needs: test
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    runs-on: [golang|node|rust]
+    steps:
+      - name: Install Node.js and Docker
+        run: |
+          apt-get update
+          apt-get install -y nodejs docker.io
+
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Get short SHA
+        id: sha
+        run: |
+          SHORT_SHA=$(git rev-parse --short HEAD)
+          echo "short_sha=${SHORT_SHA}" >> $GITHUB_OUTPUT
+
+      - name: Registry login
+        run: |
+          echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" \
+            --username "${REGISTRY_USER}" --password-stdin
+        env:
+          REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
+          REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
+
+      - name: Build Docker image
+        run: |
+          docker build --no-cache \
+            -t "${IMAGE}:${{ steps.sha.outputs.short_sha }}" \
+            -t "${IMAGE}:latest" \
+            .
+
+      - name: Push Docker image
+        run: |
+          docker push "${IMAGE}:${{ steps.sha.outputs.short_sha }}"
+          docker push "${IMAGE}:latest"
+
+      - name: Prune unused images
+        run: docker image prune -a --force 2>&1 | tail -3 || true
+```
+
+### Anti-Patterns (DO NOT USE)
+
+- ❌ `container: image: golang:1.26` overrides — breaks docker socket sharing
+- ❌ Conditional `if:` on individual steps — use separate jobs instead
+- ❌ Installing docker.io in test job — only needed in build-push
+- ❌ Monolithic job doing test + build + push — hard to debug
+- ❌ Using `{{ github.sha }}` for image tag — use short commit SHA for readability
+
+### How It Works
+
+1. **PR to feature branch** → test job runs, build-push skipped, nothing pushed
+2. **Push to main** → test runs, build-push runs after test passes, image pushed
+3. Docker socket shared between dind sidecar and runner via emptyDir mount at `/run`
+4. `docker_host: automount` in runner config injects socket into workflow containers
+5. Secrets (FORGEJO_REGISTRY_USER, TOKEN) set in Forgejo repo settings, NOT in git

k8s/infra/forgejo-runner/values.yaml

@@ -5,7 +5,7 @@ runner:
   name: golang-runner
   # Label image is what workflow steps run in (NOT the runner daemon image).
   # golang:1.26-bookworm: Debian, root, apt-get, Go, git.
-  # Install Node.js/docker in workflow steps as needed.
+  # TODO: Switch to custom image once build-runner-images.yml pushes images
   labels: "golang:docker://golang:1.26-bookworm"
   forgejoUrl: http://forgejo-gitea-http.cicd.svc.cluster.local:3000
   tokenSecret: runner-token