ci: add Blacksmith testbox workflow

.github/workflows/ci-check-testbox.yml

@@ -0,0 +1,86 @@
+name: Blacksmith Testbox
+on:
+  workflow_dispatch:
+    inputs:
+      testbox_id:
+        type: string
+        description: "Testbox session ID"
+        required: true
+  pull_request:
+    paths:
+      - '.github/workflows/**'
+
+permissions:
+  contents: read
+
+env:
+  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: "true"
+
+jobs:
+  check:
+    permissions:
+      contents: read
+    name: "check"
+    runs-on: blacksmith-16vcpu-ubuntu-2404
+    timeout-minutes: 5
+    steps:
+      - name: Begin Testbox
+        uses: useblacksmith/begin-testbox@v2
+        with:
+          testbox_id: ${{ inputs.testbox_id }}
+      - name: Checkout
+        shell: bash
+        env:
+          CHECKOUT_REPO: ${{ github.repository }}
+          CHECKOUT_SHA: ${{ github.sha }}
+          CHECKOUT_TOKEN: ${{ github.token }}
+        run: |
+          set -euo pipefail
+
+          workdir="$GITHUB_WORKSPACE"
+          auth_header="$(printf 'x-access-token:%s' "$CHECKOUT_TOKEN" | base64 | tr -d '\n')"
+
+          reset_checkout_dir() {
+            mkdir -p "$workdir"
+            find "$workdir" -mindepth 1 -maxdepth 1 -exec rm -rf {} +
+          }
+
+          checkout_attempt() {
+            local attempt="$1"
+
+            reset_checkout_dir
+            git init "$workdir" >/dev/null
+            git config --global --add safe.directory "$workdir"
+            git -C "$workdir" remote add origin "https://github.com/${CHECKOUT_REPO}"
+            git -C "$workdir" config gc.auto 0
+
+            timeout --signal=TERM 30s git -C "$workdir" \
+              -c protocol.version=2 \
+              -c "http.https://github.com/.extraheader=AUTHORIZATION: basic ${auth_header}" \
+              fetch --no-tags --prune --no-recurse-submodules --depth=1 origin \
+              "+${CHECKOUT_SHA}:refs/remotes/origin/ci-target" || return 1
+
+            git -C "$workdir" checkout --force --detach "$CHECKOUT_SHA" || return 1
+            test -f "$workdir/.github/actions/setup-node-env/action.yml" || return 1
+            echo "checkout attempt ${attempt}/2 succeeded"
+          }
+
+          for attempt in 1 2; do
+            if checkout_attempt "$attempt"; then
+              exit 0
+            fi
+            echo "checkout attempt ${attempt}/2 failed"
+            sleep $((attempt * 5))
+          done
+
+          echo "checkout failed after 2 attempts" >&2
+          exit 1
+      - name: Setup Node environment
+        uses: ./.github/actions/setup-node-env
+        with:
+          install-bun: "false"
+      - name: Run Testbox
+        uses: useblacksmith/run-testbox@v2
+        if: always()
+        env:
+          FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: "true"