[FEATURE]: Add Kilo as a native provider

[Nomadcxx]

Kilo (kilo.ai) is an AI coding platform whose gateway API is OpenAI-compatible. It'd be great to have it as a built-in provider so users can select it from the provider list and have models auto-populate — same experience as anthropic, openai, etc.

Their gateway sits at api.kilo.ai/api/gateway and exposes a /models endpoint that returns the full catalog (~346 models across anthropic, openai, google, etc). Since it's OpenAI-compatible, the implementation is mostly wiring up model discovery and auth rather than a new SDK.

There's growing interest in Kilo support across the ecosystem (simple-icons, Unity-MCP, splitrail, etc), and having native support in opencode would let Kilo subscribers use it without manual opencode.json editing.