[Bug]: New worktree threads fail in a repository with no commits #22017

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. Run git init in an empty folder. Do not commit anything.
2. Add the folder as a project in T3 Code.
3. Set new threads to start in a new worktree (Settings, or the project's default).
4. Start a new thread and send a message.

Expected behavior

The thread starts. A repository with no commits has nothing to check out, so the thread should run in the project folder, as it does for a non-Git project.

Actual behavior

Setup fails at the checkout step and no turn starts:

Workspace preparation failed during provision worktree: Git command failed in GitVcsDriver.createWorktree (<repo>): git worktree add failed

Git itself reports fatal: invalid reference: main, because main has no commit yet. Retry fails the same way. The only way forward is to make a commit or switch the thread to the main checkout.

Impact

Blocks work completely

Version or commit

main @ 673c5e3

Environment

Linux, git 2.43+. Reproduced by hand with git init -b main repo && cd repo && git worktree add -b t3/x ../wt main, which exits 128 with fatal: invalid reference: main. Not provider specific.

Logs or stack traces

$ git init -q -b main repo && cd repo
$ git worktree add -b t3/x ../wt main
fatal: invalid reference: main

Workaround

Make an initial commit, or start the thread in the main checkout.

Prior work

• [Bug]: Creating a thread in an empty git repository (no commits) crashes with "select a thread" #2148 was closed as not reproducible for the dead-route symptom. The closing comment notes that worktree creation without a commit still fails and that a local-mode fallback was not implemented.
• fix(server): new worktree threads work in repos with no commits yet #15580 added that fallback in ThreadLaunchService and was closed in favor of fix(server): t3_thread_launch refuses a new worktree whose base ref has no commit #17791.
• fix(server): t3_thread_launch refuses a new worktree whose base ref has no commit #17791 only adds a pre-launch check for the MCP t3_thread_launch tool. It accepts a repository whose HEAD has no commit and leaves that case to fix(server): new worktree threads work in repos with no commits yet #15580, so the failure above still happens on current main for every launch path.