feat: add configurable API transport format

[ferstar]

Summary

• add BUB_API_FORMAT to runtime settings
• pass the configured transport format through to republic.LLM
• document the new env var in env.example
• add a small settings test for env parsing

Verification

• uv run pytest -q tests/test_builtin_settings.py

[ferstar]

This is also meant to be a cleaner alternative to #67. Instead of introducing a use_responses boolean that hard-codes one transport decision into app config, this keeps the setting aligned with Republics actual api_format surface (completion | responses | messages), which is easier to extend and avoids baking a shim-specific compatibility flag into Bub.