fix(fleet): consume model_hint + fix socket-default trap (stand-up fixes) (#627)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

packages/mosaic/framework/systemd/user/mosaic-agent@.service

@@ -8,13 +8,15 @@ PartOf=mosaic-tmux-holder.service
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+# No default MOSAIC_TMUX_SOCKET: an absent roster socket means the literal
+# default tmux socket (no -L). The per-agent .env sets it when the roster names
+# one; otherwise it stays unset and start-agent-session.sh uses the default socket.
 Environment=MOSAIC_AGENT_NAME=%i
 Environment=MOSAIC_AGENT_RUNTIME=pi
 Environment=MOSAIC_AGENT_WORKDIR=%h
 EnvironmentFile=-%h/.config/mosaic/fleet/agents/%i.env
 ExecStart=/bin/bash %h/.config/mosaic/tools/fleet/start-agent-session.sh %i
-ExecStop=-/bin/bash -lc 'tmux -L "${MOSAIC_TMUX_SOCKET:-mosaic-factory}" kill-session -t "=%i"'
+ExecStop=-/bin/bash -lc 'if [ -n "${MOSAIC_TMUX_SOCKET:-}" ]; then tmux -L "$MOSAIC_TMUX_SOCKET" kill-session -t "=%i"; else tmux kill-session -t "=%i"; fi'
 
 [Install]
 WantedBy=default.target