Allows users to right-click "+ New thread" to create threads from specific git branches, with optional worktree creation for isolated work. Includes branch sorting (current/default first), graceful handling of non-git repos, and ability to initialize git repos.

- New git IPC contract (listBranches, createWorktree, removeWorktree, init)
- Branch context menu with worktree sub-option in Sidebar
- Thread model extended with branch/worktreePath fields
- Session cwd uses worktree path when available
- Branch sorting: current > main/master > alphabetical

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>