ci(security): SHA-pin all actions and add harden-runner (audit)

.github/workflows/ci.yml

@@ -26,11 +26,16 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - name: Harden the runner
+        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
+        with:
+          egress-policy: audit
+
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
 
       - name: Setup pnpm
-        uses: pnpm/action-setup@v5
+        uses: pnpm/action-setup@fc06bc1257f339d1d5d8b3a19a8cae5388b55320 # v5.0.0
         with:
           version: 10.34.3
 
@@ -39,7 +44,7 @@ jobs:
       # install below pulls better-sqlite3's prebuilt binary for the correct ABI,
       # which is what makes the server suite runnable in CI.
       - name: Setup Node.js
-        uses: actions/setup-node@v5
+        uses: actions/setup-node@a0853c24544627f65ddf259abe73b1d18a591444 # v5.0.0
         with:
           node-version: 20
           cache: pnpm