Fix: echo request_id in agent_prompt ack so clients recognize it

The agent_prompt handler was the only command that did not echo the
client's request_id in its acknowledgment. The Telegram client's
await_command_result filters command_result messages by request_id, so
the ack was always skipped and every free-form reply failed after a 20s
timeout with "relay did not acknowledge agent_prompt" — even though the
prompt was already delivered to the agent.

Read and echo request_id in the success ack and both error responses,
matching the existing respond/send_keys handlers.

Fixes #40

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>