fix(git-sync): use reset --mixed so commit parent is FETCH_HEAD

git read-tree only updated the index, leaving HEAD on our previous
local commit. The push commit's diff then included all remote changes
outside subPath as if we made them. reset --mixed also moves HEAD to
FETCH_HEAD, so the new commit's parent is the remote HEAD and the diff
shows only the managed area changes.

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

services/web/app/src/Features/GitSync/GitSyncHandler.mjs

@@ -173,7 +173,9 @@ async function pushToRemote(
   // read-tree updates the index without checking out any files, so no blobs are downloaded.
   try {
     await spawnGit(['fetch', '--filter=blob:none', 'origin', branch], repoDir)
-    await spawnGit(['read-tree', 'FETCH_HEAD'], repoDir)
+    // --mixed: moves HEAD to FETCH_HEAD + updates index, no working tree change.
+    // This makes FETCH_HEAD the parent of our next commit so the diff only shows subPath changes.
+    await spawnGit(['reset', '--mixed', 'FETCH_HEAD'], repoDir)
   } catch {
     // Empty remote or first push — proceed with empty index
   }