fix(picker): show notification instead of empty window when no sessions match

[hermandavid]

Summary

• When using fzf as a picker and selecting child sessions with none available, an empty fzf window would open. Snacks handles this gracefully with a built-in "no results" message, but fzf does not.
• Add an early return in core.select_session() that checks for empty results before opening the picker, showing a contextual notification instead ("No child sessions found" or "No sessions found").

Details

This follows the same pattern already used by the history, reference, and MCP pickers, which all guard against empty item lists before invoking base_picker.pick().

The fix is in core.select_session() rather than in the picker layer, since this is the call site that knows whether we're filtering by parent (child sessions) or listing all sessions, and can provide the appropriate message.

[sudo-tee]

Thanks for the PR

Also looks good to me