When selecting a branch that lives in a different worktree, the store
updated the thread's worktreePath but the running session kept its
original cwd. The agent continued operating in the old directory.

- SET_THREAD_BRANCH now clears the session when worktreePath changes
- BranchToolbar stops the server-side session before dispatching the
  branch change, so the next message creates a fresh session with the
  correct cwd

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>