BJ1919810/dsh-session-mail

Agent-to-agent mail between DSH sessions on the same host. `send_session_message` delivers after the target's turn or injects into the running one, stamps the sender id itself and rejects forged From headers; every message carries an id, so replies quote it with replyTo, a thread stops the same task being queued twice, and supersedes recalls messages that are still pending. `read_session_info` reads a peer read-only: title, last reply, the files of its last present call, and busy/queue state.