Pin GitHub Actions to SHA for security

Update official GitHub Actions (actions/*, github/*) to use pinned
commit SHAs instead of version tags. This satisfies zizmor's
unpinned-action-reference security check.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: oschwald

.github/workflows/release.yml

@@ -18,7 +18,7 @@ jobs:
     permissions:
       id-token: write
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
 

.github/workflows/rubocop.yml

@@ -12,7 +12,7 @@ jobs:
   rubocop:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
 

.github/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
           - os: windows-latest
             version: jruby
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           submodules: true
           persist-credentials: false

.github/workflows/zizmor.yml

@@ -15,7 +15,7 @@ jobs:
       security-events: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false