fix: prevent live branch sync from clearing thread metadata on detached HEAD

When the repository enters detached HEAD state (e.g., during rebase,
bisect, or manual commit checkout), gitStatus.branch becomes null.
resolveLiveThreadBranchUpdate would return { branch: null }, causing
persistThreadBranchSync to permanently clear the thread's branch
metadata.

Add a guard to skip syncing when gitStatus.branch is null, since
detached HEAD is a transient state and the stored branch association
should be preserved.

Author: cursoragent

apps/web/src/components/GitActionsControl.logic.ts

@@ -316,7 +316,7 @@ export function resolveLiveThreadBranchUpdate(input: {
   threadBranch: string | null;
   gitStatus: GitStatusResult | null;
 }): { branch: string | null } | null {
-  if (!input.gitStatus) {
+  if (!input.gitStatus || !input.gitStatus.branch) {
     return null;
   }