When no agent is active, '/queue <prompt>' previously fell through
dispatch with its raw text intact instead of being treated as a
normal prompt. Rewrite event.text to the bare payload (mirroring the
/steer no-active-agent path just below) and return a usage hint when
the payload is empty.
Salvaged from PR #29290 (queue half only — the /footer mid-run
dispatch half already landed on main via #65521).
