v0.3.20

Fixes:
- Custom providers now accepted by API endpoints — set_provider_url and set_provider_key no longer reject unknown provider names (#321)
- Custom providers auto-registered in catalog with derived env var name
- channel_send tool now supports image_url, file_url, and filename params for media sending (#302)
- New send_channel_media kernel method routes Image/File content to channel adapters
- Strip <think>...</think> blocks from model output in WS, HTTP, and OpenAI-compat endpoints (#330)