docs: plan durable tmux fleet install (#557)

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

@@ -0,0 +1,20 @@
+[Unit]
+Description=Mosaic tmux fleet agent %i
+Documentation=https://git.mosaicstack.dev/mosaicstack/stack
+Requires=mosaic-tmux-holder.service
+After=mosaic-tmux-holder.service
+PartOf=mosaic-tmux-holder.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+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"'
+
+[Install]
+WantedBy=default.target