fix: Pin all GitHub Actions to full commit SHAs (13 files)

Security compliance - SHA pinning for all actions.

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

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

.github/workflows/blackroad-auto-merge.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: 📥 Checkout Repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
 
       - name: 🔍 Get PR Details
         id: pr