Move agent discovery out of `agent.sh` so the picker runs on every new
pane, including manual splits. Set the agent session's default-command
to agent-cmd.sh and re-apply session options on every invocation so
changes propagate to live sessions.
Add agent-cmd.sh wrapper that detaches the client when all panes in the
current window have exited. This allows multiple agent windows (one per
directory) while automatically returning to the shell when done with a
particular window's agents.
- remain-on-exit keeps panes alive after agent exits to check status
- Wrapper counts running panes and detaches when it's the last one
