opencode provider: unhelpful ENOENT when the opencode CLI is not on PATH

Follow-up from #856. The attach path was removed in #1445, but createOpencodeServer() in packages/ai/providers/opencode-sdk.ts (~:93-98) has no try/catch around the spawn: when the opencode CLI is missing, the raw uv_spawn ENOENT propagates through ensureServer()'s rethrow with no PATH probe and no actionable hint. Wrap the spawn and surface a clear message ('opencode CLI not found on PATH - install it or disable the opencode provider'), ~10 lines. Reported shape: Windows, Ask AI with the opencode provider selected.