[Bug]: Threads an agent starts in the project folder never show their branch in the sidebar #31177

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Have a project whose folder is a git repository checked out on a branch, for example fix/checkout-total.
2. Start a thread in that project's folder (no worktree) the way an agent does: call the t3_thread_launch MCP tool without workspaceStrategy, or run t3 thread start <project> "message".
3. Look at the thread's row in the sidebar.

Expected behavior

The row shows the branch on its second line, as it does for a thread started from the composer in the same folder.

Actual behavior

The row has no branch line. The thread is saved with branch: null, and the sidebar row only shows a saved branch. The composer passes the checkout's branch when it launches a project-folder thread (packages/client-runtime/src/operations/commands.ts), but t3_thread_launch defaults to { type: "root" } with no branch, and ThreadLaunchService records the branch it was given.

The thread details card still shows the live branch, so the two disagree. No agent tool can set a branch on an existing thread afterwards; the only way back is enabling Settings → Appearance → Composer context and picking the branch by hand.

Impact

Minor bug or occasional failure

Version or commit

T3 Code (Nightly) 0.1.8-nightly.20261104.4179; the same code is on main @ a21d788

Environment

macOS 27.2, T3 Code (Nightly) desktop app

Screenshots, recordings, or supporting files

A thread started with t3 thread start on main (dev server, empty data, demo repo on fix/checkout-total). The row has no branch line:

Workaround

Turn on Settings → Appearance → Composer context, then pick the branch in the branch picker under the thread's composer.