fix: improve concurrency group to prevent workflow conflicts

Update concurrency group to use PR number for better isolation.
This prevents new commits from cancelling unrelated workflow runs.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ScriptedAlchemy

.github/workflows/on-pull-request.yml

@@ -2,7 +2,7 @@ name: on pull request
 on: pull_request
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true
 
 env: