The cap is shared across CLI, desktop/TUI and the messaging gateway, so the
surface that gets rejected is rarely the one holding the slots. The rejection
read "Hermes is at the active session limit (5/5). Try again when another
session finishes." while every slot was an idle desktop tab, which took
filesystem access to work out.
Name the holders in the message, and show slot usage plus each holder in
`hermes status`. Both are inert when max_concurrent_sessions is unset, which
is the default. The gateway's duplicate copy of the message now reuses the
shared helper.
