ci: harden workflows against token exfiltration

Disable checkout credential persistence so repository-controlled code
run by npm ci/test cannot read GITHUB_TOKEN from .git/config, and pin
actions to full commit SHAs to guard against mutable-tag supply-chain
attacks.

.github/workflows/ci.yml

@@ -14,10 +14,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        with:
+          persist-credentials: false
 
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
         with:
           node-version: 24
 

.github/workflows/npm-publish.yml

@@ -14,9 +14,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           fetch-depth: 0
+          persist-credentials: false
 
       - name: Verify tag is on main
         run: |
@@ -26,7 +27,7 @@ jobs:
           }
 
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
         with:
           node-version: 24
           registry-url: 'https://registry.npmjs.org'