🔒️(ci) set persist-credentials: false on actions/checkout

By default, `actions/checkout` saves the job's auth token
(`GITHUB_TOKEN` or the provided PAT) in the local git config so
later steps can run authenticated git commands. That token then
stays on disk for the rest of the job, where it can leak:

* If an artifact upload includes the checkout directory, the token
  is packaged with it and anyone with artifact access can extract
  it. On public repos that is anyone, and the token can be used
  while the job is still running ("ArtiPACKED", flagged by
  `zizmor` as `artipacked`).
* Any later step, third-party action, or build dependency can read
  the token from the git config, which widens the impact of a
  supply-chain compromise.

None of our workflows need authenticated git after checkout, so
disable credential persistence. If a step needs to push in

.github/workflows/ci.yml

@@ -88,6 +88,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Create writable /data
         run: |
@@ -159,6 +161,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Install ffmpeg
         run: |
@@ -186,6 +190,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Install dependencies
         run: cd src/frontend/ && npm ci --ignore-scripts
@@ -203,6 +209,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Install Node.js
         uses: actions/setup-node@249970729cb0ef3589644e2896645e5dc5ba9c38 # v6.5.0
@@ -225,6 +233,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Install dependencies
         run: npm ci --ignore-scripts
@@ -246,6 +256,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
+        with:
+          persist-credentials: false
 
       - name: Install dependencies
         run: npm ci --ignore-scripts