Multi-repo on v2: no way to pick a base branch per repository for a New worktree run #345

On v2 multi-repo (#338), every repository of a multi-repo isolated run starts from its own default branch, with no way to pick another. The web composer has no per-repository branch control. On mobile, a project folder that is not a repository shows "Choose branch" with "No branches available", and Start task stays disabled, so the run cannot start at all. The pre-v2 branch had a branch per repository (#113, #295).

Repro: api has a release branch. Start a New worktree thread in team and try to start API from release.

Expected: a repositories menu beside the branch controls on web and mobile, with a branch list per repository and a way back to the default. The worktree builder starts each repository from its pick, and t3_thread_launch and scheduled tasks can pass the same picks.