Live model enumeration for Pi via `pi --list-models`

Pi is the only supported agent whose CLI can enumerate models, so wire a real ListModels that shells out to pi --list-models (output is on stderr, so capture combined output) and parses the provider/model table into "provider/model" ids. On failure it returns an error and the picker falls back to Default + Custom.

Other agents are unchanged: claude-code keeps its real aliases; codex and gemini have no enumeration command, so they show only Default + Custom. Refreshed the ModelLister doc comment to match.