Admin-merge: fix(pr-validation): add head_ref fetch to checkout step (PRI-1588)

Merges PR #197 with conflict resolution. The PR adds a missing
`git fetch origin head_ref` step so the PR head SHA is available
before checkout. Conflict arose because PR 195 also touched this file
(changed runs-on label). Both changes are preserved.

Admin-merge authorized by PRI-1590 — bootstrap CI fix bypasses
branch-protection CI requirement by board policy.

.github/workflows/pr-validation.yaml

@@ -14,6 +14,7 @@ jobs:
         run: |
           git clone --depth=1 "https://x-access-token:${{ secrets.GITEA_TOKEN }}@git.farh.net/${{ github.repository }}.git" .
           git fetch origin "${{ github.base_ref }}" --depth=1
+          git fetch origin "${{ github.head_ref }}" --depth=1
           git checkout "${{ github.sha }}"
 
       - name: Install actionlint