chore: Sync Platform on Pull Requests Only (#757)

## Summary

Run push_to_platform on pull request merge only. Also use default
message


## Checklist
- [ ] Added Unit Tests
- [ ] Covered by existing CI
- [ ] Integration tested
- [ ] Documentation update



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Updated workflow to run only after a pull request is merged into the
main branch, instead of on every push.
- Adjusted the commit message behavior for subtree updates to use the
default message.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: chewy-zlai

.github/workflows/push_to_platform.yaml

@@ -1,13 +1,14 @@
 name: Push to Platform
 
 on:
-  push:
-    branches:
-      - main
+  pull_request:
+    types: [closed]
+    branches: [main]
 
 jobs:
   subtree-pull:
     runs-on: ubuntu-latest
+    if: github.event.pull_request.merged == true
 
     steps:
       - name: Checkout platform repo
@@ -50,7 +51,7 @@ jobs:
           git remote add chronon [email protected]:zipline-ai/chronon.git || true
           
           git fetch chronon main
-          git subtree pull --prefix=chronon chronon main --message="chore: update chronon subtree"
+          git subtree pull --prefix=chronon chronon main
 
       - name: Push changes to platform
         run: git push origin main