The Start() function was returning success even if the pane died during
initialization (e.g., if Claude failed to start). This caused the caller
to get a confusing "getting pane" error when trying to use the session.
Now Start() verifies the session is still running at the end, returning
a clear error message if the session died during startup.
Fixes: gt-0cif0s
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>