Add provider layer (src/providers/) with LLMService interface,
complexity-based model router (light/standard/heavy tiers),
provider registry, Anthropic and Ollama implementations.
Update settings UI with provider config and model routing.
Update drafter, context, commands for multi-provider support.
Migrate settings from old apiKey/model to new per-provider format.
81 new provider tests (244 total, all passing).