v0.6.0: remove model group aliases, fix vision group

- remove model_group_alias (was silently broken — maps to one model, not a list)
- remove group-level fallbacks (fast/smart/vision) that depended on aliases
- add local-ollama-moondream to vision group in config
- fix README: update usage examples to use actual model names
- fix README: Speaches description includes TTS