Add Phase 3 CI smoke workflow (host runner toolchain validation)

Validates the bundled node/git/docker toolchain and mounted Docker socket
by running directly in the custom-gitea-runner container via the host label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci-smoke.yml

@@ -0,0 +1,34 @@
+name: CI Smoke
+
+# Phase 3 validation: exercises the bundled toolchain on the custom runner.
+# Runs on the `host` label so steps execute directly in the custom-gitea-runner
+# container, using its bundled node/git/docker and the mounted Docker socket.
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  toolchain:
+    runs-on: host
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Show bundled tool versions
+        run: |
+          echo "node   : $(node --version)"
+          echo "npm    : $(npm --version)"
+          echo "git    : $(git --version)"
+          echo "docker : $(docker --version)"
+
+      - name: Verify working directory
+        run: |
+          echo "Repo: $(pwd)"
+          git log --oneline -3
+
+      - name: Verify Docker socket access
+        run: |
+          docker info --format 'Docker daemon reachable: server {{.ServerVersion}}'
+          docker images --format '{{.Repository}}:{{.Tag}}' | head -5