Sync task completion was fragile — detecting premature stability during
brief idle periods between tool calls. Now mirrors opencode's native
SessionPrompt.loop() logic: checks assistant finish reason is terminal
(not tool-calls/unknown) and assistant.id > user.id.
Also switches sync prompt sender from blocking HTTP (promptSync) to
async fire-and-forget (promptAsync) to avoid JSON parse errors in ACP.
11f587194f