fix: getHead returns default Branch(main) for empty repos git-remote-cospan calls getHead before the initial push to determine the default branch. Returning 404 on non-existent vcs stores prevented any initial push from succeeding. Return HeadState::Branch("main") as the empty-repo default so clients can proceed with push.

1818     Query(params): Query<GetHeadParams>,
1919 ) -> Result<Json<serde_json::Value>, NodeError> {
2020     let store = state.store.lock().await;
21+
22+    // Return an empty-repo default (Branch("main")) when the vcs store
23+    // doesn't exist yet. git-remote-cospan calls getHead before the first
24+    // push to determine the default branch; returning 404 would prevent
25+    // any initial push from ever succeeding.
2126     let head = store
2227         .get_head(&params.did, &params.repo)
23-        .map_err(|_| NodeError::RepoNotFound {
24-            did: params.did.clone(),
25-            name: params.repo.clone(),
26-        })?;
28+        .unwrap_or_else(|_| panproto_core::vcs::HeadState::Branch("main".to_string()));
2729 
2830     // Flat format matches panproto-xrpc NodeClient expectations.
2931     let head_json = match head {