feat(fleet): add durable tmux fleet poc

packages/mosaic/framework/systemd/user/mosaic-tmux-holder.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Mosaic tmux fleet holder
+Documentation=https://git.mosaicstack.dev/mosaicstack/stack
+After=default.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+Environment=MOSAIC_TMUX_SOCKET=mosaic-factory
+Environment=MOSAIC_TMUX_HOLDER=_holder
+ExecStart=/bin/bash -lc 'tmux -L "$MOSAIC_TMUX_SOCKET" has-session -t "=${MOSAIC_TMUX_HOLDER}:0.0" 2>/dev/null || tmux -L "$MOSAIC_TMUX_SOCKET" new-session -d -s "$MOSAIC_TMUX_HOLDER" "while true; do sleep 3600; done"'
+ExecStop=-/bin/bash -lc 'tmux -L "$MOSAIC_TMUX_SOCKET" kill-server'
+
+[Install]
+WantedBy=default.target