sendChatMessage was awaiting the full agentManager.spawn() which
includes worktree creation, ~50 writeFileSync calls for input files,
credential setup, and process spawning. This blocked the Node.js event
loop long enough to cause ECONNREFUSED on the SSE connection.
Now the mutation returns immediately after storing the user message
and creating the task. The heavy spawn work runs in a detached promise.
On failure, a system message is stored so the UI can display the error.
8ee02f6013