docs(git): explain handoff pack filesystem requirement

Generated-By: PostHog Code
Task-Id: b73fc361-26b5-4bae-8c3c-6e4682d0db91

Author: tatoalo

packages/git/src/handoff.ts

@@ -547,6 +547,7 @@ export class GitHandoffTracker {
     git: GitClient,
     checkpointId: string,
   ): Promise<string> {
+    // Git stages packs in the object store, so the destination must share its filesystem.
     const gitCommonDir = await this.resolveGitCommonDir(git);
     return mkdtemp(
       path.join(gitCommonDir, `posthog-code-handoff-${checkpointId}-`),