Harden GitHub Actions workflows (#220)

* ci: add GitHub Actions audit job

* ci: configure dependabot for batched github-actions updates

* ci: pin GitHub Actions to SHA hashes

* ci: suppress zizmor cache-poisoning warnings

* ci: scope permissions to jobs and disable credential persistence

Author: flavorjones

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+    schedule:
+      interval: weekly
+    cooldown:
+      default-days: 7

.github/workflows/ci.yml

@@ -15,39 +15,72 @@ on:
     branches:
       - '*'
 
+permissions: {}
+
 jobs:
   rubocop:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
       - run: rm Gemfile.lock
-      - uses: ruby/setup-ruby@v1
+      - uses: ruby/setup-ruby@d45b1a4e94b71acab930e56e79c6aa188764e7f9 # zizmor: ignore[cache-poisoning] -- cached gems are used for testing, not release artifact generation # v1.316.0
         with:
           ruby-version: "4.0"
           bundler-cache: true
       - run: bundle exec rubocop
 
+  lint-actions:
+    name: GitHub Actions audit
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          persist-credentials: false
+
+      - name: Run actionlint
+        uses: rhysd/actionlint@393031adb9afb225ee52ae2ccd7a5af5525e03e8 # v1.7.11
+
+      - name: Run zizmor
+        uses: zizmorcore/zizmor-action@71321a20a9ded102f6e9ce5718a2fcec2c4f70d8 # v0.5.2
+        with:
+          advanced-security: false
+
   cruby:
     strategy:
       fail-fast: false
       matrix:
         ruby: ["2.7", "3.0", "3.1", "3.2", "3.3", "3.4", "4.0", "ruby-head", "truffleruby-head", "jruby-9.4", "jruby-10.0", "jruby-head"]
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
       - run: rm Gemfile.lock
-      - uses: ruby/setup-ruby@v1
+      - uses: ruby/setup-ruby@d45b1a4e94b71acab930e56e79c6aa188764e7f9 # zizmor: ignore[cache-poisoning] -- cached gems are used for testing, not release artifact generation # v1.316.0
         with:
           ruby-version: ${{matrix.ruby}}
           bundler-cache: true
       - run: bundle exec rake
 
   cruby-nokogiri-system-libraries:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
       - run: rm Gemfile.lock
-      - uses: ruby/setup-ruby@v1
+      - uses: ruby/setup-ruby@d45b1a4e94b71acab930e56e79c6aa188764e7f9 # v1.316.0
         with:
           ruby-version: "4.0"
       - name: Install nokogiri with system libraries