security(ci): harden sync-cnb.yml — permissions, checkout v4, narrow trigger

- Add explicit permissions: contents: read (least-privilege)
- Bump actions/checkout@v3 → @v4
- Narrow trigger from on: [push] to on: push: branches: [main] + tags: ['v*']

Matches the hardening convention used by every other workflow in the repo.

.github/workflows/sync-cnb.yml

@@ -1,12 +1,18 @@
 name: Sync to CNB
 
-on: [push]
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+
+permissions:
+  contents: read
 
 jobs:
   sync:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0