Add GitHub Actions run link to commit messages

When running in GitHub Actions, append a link to the workflow run
in the commit message body for easier debugging and traceability.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Phlogistique

update-pr-stack.sh

@@ -114,6 +114,11 @@ update_direct_target() {
         log_cmd git merge --no-edit -s ours SQUASH_COMMIT
         log_cmd git update-ref MERGE_RESULT "HEAD^{tree}"
         COMMIT_MSG="Merge updates from $BASE_BRANCH and squash commit"
+        if [[ "${GITHUB_ACTIONS:-}" == "true" ]]; then
+            COMMIT_MSG="$COMMIT_MSG
+
+See $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID"
+        fi
         CUSTOM_COMMIT=$(log_cmd git commit-tree MERGE_RESULT -p BEFORE_MERGE -p "origin/$MERGED_BRANCH" -p SQUASH_COMMIT -m "$COMMIT_MSG")
         log_cmd git reset --hard "$CUSTOM_COMMIT"
     fi