Respect disabled shipped personas

[Martinnn674]

Summary

• Return an empty persona list when use_shipped_personas is disabled and no custom personas are configured.
• Add a settings unit test for the empty-persona case from the issue.

Fixes #31

Verification

• uv run --with pytest --with pydantic --with pyyaml pytest tests/test_settings.py -q
• uv run --with pytest --with pydantic --with pyyaml --with litellm --with rich --with prompt_toolkit pytest tests/ -q
• uv run --with flake8 flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

[rosspeili]

Thanks for the focused fix @Martinnn674.

Before merge, could you also take care of these small UX/docs pieces in the same PR in follow up commits if you want?

1. CLI hint, when get_default_personas() returns an empty list, show a short message in the wizard (e.g. that no default personas are configured and the user should add personas: in rooms.settings.yaml or create a custom agent). Right now it still prints “Available Default Personas:” with nothing listed; the “You must have at least 1 agent!” guard works, but a clearer hint would help.
2. get_default_personas() docstring, document that an empty list is returned when use_shipped_personas: false and no custom personas are defined.
3. Docs, one line in docs/EXAMPLES.md or docs/ARCHITECTURE.md noting that use_shipped_personas: false requires YAML personas and/or custom agents in the wizard.

Once those are in and CI is green, this should be good to merge. Thanks again for picking up a good first issue and tagging along!