feat(opencode): add KeiRouter plugin

[bobbyunknown]

What

Add a local OpenCode plugin package for KeiRouter model discovery.

Why

KeiRouter needs OpenCode integration without publishing to npm first. The plugin lets OpenCode fetch KeiRouter models from the local OpenAI-compatible /v1/models endpoint and cache them client-side.

How

• Added @keirouter/opencode-plugin as a small standalone package.
• Implements OpenCode auth, provider, and legacy config hooks.
• Fetches only GET /v1/models with Bearer auth.
• Adds in-memory TTL caching so repeated model lookups avoid extra network calls.
• Uses KeiRouter default local URL: http://127.0.0.1:20180.
• Added unit tests for URL handling, auth, model mapping, fetch behavior, and cache reuse.

Checklist

• make test passes
• make vet passes
• npm run lint and npm run typecheck pass (if frontend changed)
• Documentation updated (if applicable)
• Config example updated (if applicable)

Additional verification:

• npm test passes in @keirouter-opencode-plugin
• npm run build passes in @keirouter-opencode-plugin
• Live local fetch against KeiRouter /v1/models succeeds

[mydisha]

Thanks for the plugin contribution, will merge to main @bobbyunknown