add cli-sync workspace snapshot/load scripts (#9322)

* feat(fixtures): add cli-sync workspace snapshot/load scripts

* fix(fixtures): address review nits (env var password, mktemp, dead refs)

* fix(fixtures): address CI review (SIGPIPE, JSON escaping, doc/code drift)

.github/workflows/check-empty-fixture.yml

@@ -0,0 +1,19 @@
+name: Check fixture is empty
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "fixtures/**"
+  pull_request:
+    paths:
+      - "fixtures/**"
+
+jobs:
+  check-empty-fixture:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Ensure fixtures/cli-sync/ has no committed snapshot
+        run: bash fixtures/check-empty.sh