farm: built-in default API base URL is stale (api.opencode.ai/v1 → 404) #90

Found during the docs-site-mvp /ca:sprint --farm run (2026-06-18, Windows).

ENV.defaultApiBaseUrl in plugins/ca/tools/farm.ts is https://api.opencode.ai/v1. That host/path now returns HTTP 200 with body Not Found for both /models and /chat/completions, so every worker fails with non-JSON response: ... "Not Found" is not valid JSON.

The live OpenCode Zen OpenAI-compatible endpoint is https://opencode.ai/zen/v1 (verified: /models and /chat/completions both 200, models owned_by: "opencode").

Workaround used: set FARM_API_BASE_URL=https://opencode.ai/zen/v1 and plan.meta.apiBaseUrl.

Perma-fix candidates:

• Update defaultApiBaseUrl to https://opencode.ai/zen/v1.
• Treat a non-JSON / Not Found body as an endpoint-config error with a clear message ("check FARM_API_BASE_URL"), not an opaque worker error.
• Update includes/farm.md (it documents the old default).