ci: guard empty BRANCH and deduplicate dist-dirty PR comment

- commit-dist: add explicit check after git branch -r derivation —
  if BRANCH is empty (shallow clone edge case or post-force-push race),
  fail with a clear error instead of a cryptic git error
- comment-dist-dirty: use --edit-last so repeated fork PR pushes that
  consistently miss the dist/ update edit the existing bot comment
  instead of spamming new ones; fall back to a new comment if none exists

Author: ricogu

.github/workflows/build-and-test.yml

@@ -113,6 +113,10 @@ jobs:
           > git push
           > ```
           EOF
+          gh pr comment "${{ github.event.pull_request.number }}" \
+            --repo "${{ github.repository }}" \
+            --body-file /tmp/comment.md \
+            --edit-last || \
           gh pr comment "${{ github.event.pull_request.number }}" \
             --repo "${{ github.repository }}" \
             --body-file /tmp/comment.md
@@ -159,6 +163,10 @@ jobs:
           PR_HEAD=$(git rev-parse HEAD^2)
           BRANCH=$(git branch -r --contains "$PR_HEAD" --format='%(refname:short)' \
             | grep '^origin/' | grep -v 'origin/HEAD' | head -1 | sed 's|^origin/||')
+          if [[ -z "$BRANCH" ]]; then
+            echo "::error::Could not determine PR branch from git history."
+            exit 1
+          fi
           git checkout -B "$BRANCH" "origin/$BRANCH"
           cp -r /tmp/dist-artifact/. .github/actions/core/dist/
           git add .github/actions/core/dist/