[FEATURE]: Add Keenable as an optional websearch backend #1727

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Could Keenable be added as an optional backend in packages/opencode/src/tool/websearch.ts and packages/core/src/tool/websearch.ts, next to Exa and Parallel? I work with Keenable, so I'd write and maintain it.

It would stay opt-in. The current default provider would not change.

With no key it would call POST https://api.keenable.ai/v1/search/public with an X-Keenable-Title: Altimate header (1,000 requests per hour per IP, docs). An optional KEENABLE_API_KEY would use POST /v1/search and lift that hourly cap. The free tier is 100k requests a month. There is also a keyless MCP at https://api.keenable.ai/mcp-public. Results are title, url and snippet.

If you want it, I'll open a small PR that links this issue, with tests in the existing websearch suites.