ci: guard against private cloud service code in public repo

Vestige Cloud is split: the public client (a thin HTTP sync backend that
only moves encrypted bytes) belongs here, but the hosted service — billing,
sync-key->namespace mapping, per-user isolation, Lemon Squeezy webhooks,
transactional email — must live only in the private repo.

Add scripts/check-no-private-cloud.sh, which git-greps tracked files for
distinctive private-service signatures (service crate identity, module
headers, billing/provider internals, server-side sync-key mapping SQL). The
patterns are chosen so the legitimate public client — including its
VESTIGE_CLOUD_* client env vars — does not match.

Wired into CI via guard-no-private-cloud.yml on push/PR. Verified both
directions: passes on the clean repo, fails (naming the markers) when real
private webhook.rs/keys.rs are introduced.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/guard-no-private-cloud.yml

@@ -0,0 +1,25 @@
+name: Guard — No Private Cloud Code
+
+# Fails if private Vestige Cloud *service* code (billing, sync-key/namespace
+# mapping, Lemon Squeezy webhooks, transactional email) ever lands in this
+# public repo. The public cloud *client* is allowed and does not trip this.
+on:
+  push:
+    branches: [main, feat/cloud-sync-mvp]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  guard:
+    name: No private cloud service code
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Scan for private cloud service markers
+        run: ./scripts/check-no-private-cloud.sh