Pin workflow actions and document live-edit script trust (#755)

Pin existing action versions to verified commit SHAs, restrict CI to a read-only repository token, and document the optional live-edit validation script. Preserve required sync/sheriff permissions and existing runtime behavior. Related to #480; extension permission assessment remains separate.

AI assistance: Codex, under maintainer direction.

.github/workflows/sync-generated-output.yml

@@ -49,7 +49,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v7
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           fetch-depth: 0
           # Optional PAT or GitHub App token. With the default GITHUB_TOKEN,
@@ -57,12 +57,12 @@ jobs:
           token: ${{ secrets.SYNC_GENERATED_OUTPUT_TOKEN || github.token }}
 
       - name: Setup Node
-        uses: actions/setup-node@v7
+        uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
         with:
           node-version: 24
 
       - name: Setup Bun
-        uses: oven-sh/setup-bun@v2
+        uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
           bun-version: latest