Add etserver admin commands to list and close disconnected sessions

[epsalmond]

Problem

et --list shows only the client side saved sessions. There is no server-side way to manage.

This complements #779, #819, and #820: those cover client-side programmatic session control, while this request is for the running etserver daemon itself. It follows #793.

Proposed interface

etserver --list
etserver --close-all-idle 30   # require 30 minutes disconnected
etserver --close id

Listing fields

Show the data the server currently has: session/client ID, UID, PTY state, whether an ET client is connected, and disconnected age.

Acceptance criteria

• Tests cover disconnect-age tracking, the threshold boundary, and a reconnect racing closure.
• Documentation distinguishes server inventory from et --list's local saved-session list.