Datasette-LLM Adds Plugin Hook For Model Purposes

Datasette-LLM 0.1a1 was released as a base plugin that exposes LLM models for other Datasette plugins to use, including datasette-enrichments-llm. It adds register_llm_purposes() and get_purposes() hooks and lets plugin authors map named purposes (for example, 'enrichment' → GPT-5.4-nano, 'sql-assist' → Sonnet 4.6), call llm.model(purpose='enrichment'), and enable admin UIs to assign models to purposes.