Split benchmark concurrency group by event type

Bot review events (pull_request_review) were racing against and
cancelling legitimate push-triggered (pull_request) benchmark runs
via the shared concurrency group.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/bench.yml

@@ -7,7 +7,7 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
   cancel-in-progress: true
 
 jobs: