fix: checkout release tag ref in check-release-notes job

Ensures the release-notes check validates the tagged tree, not the
default branch HEAD. Without this, manually triggered runs could
validate the wrong commit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

.github/workflows/release.yml

@@ -94,6 +94,8 @@ jobs:
     steps:
       - name: Checkout Source
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+        with:
+          ref: ${{ inputs.git-tag }}
 
       - name: Set up Python
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0