Moving a thread to another worktree detaches its provider session, and on
V2 the server does that itself for any worktree change. Picking a ref from
another worktree mid-run therefore interrupted the run with no warning.

While Stop is offered for the thread, the picker disables refs that would
move it to another worktree and shows why. Enter and a run that starts
during a checkout are rechecked against the latest thread state.

Refs #14245

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>