chore(ci): Pin GitHub Actions to commit SHAs

Pin GitHub Actions references to full commit SHAs instead of version
tags to reduce supply chain risk from tag retargeting.

Update actions/checkout to v6.0.2, actions/setup-python to v6.2.0,
actions/setup-node to v6.3.0, actions/stale to v10.2.0, and
dessant/lock-threads to v6.0.0.

.github/workflows/claude-code-review.yml

@@ -21,7 +21,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 1