Set up custom merge strategy for pyproject.toml

Added .gitattributes rule and Git configuration to always prefer our fork's version of pyproject.toml during merges. Ensures smoother upstream syncs by preventing conflicts in this file.

Author: urban233

.gitattributes

@@ -1 +1,3 @@
 *.pkl binary
+# Always keep our fork's version of pyproject.toml
+pyproject.toml merge=ours

.github/workflows/sync-upstream.yaml

@@ -46,6 +46,10 @@ jobs:
       - name: Checkout unstable branch
         run: git checkout ${{ env.UNSTABLE_BRANCH }}
 
+      - name: Configure Git merge driver
+        run: |
+          git config merge.ours.driver true
+
       - name: Merge upstream/${{ env.UPSTREAM_BRANCH }} into unstable
         run: |
           set -e