Investigate running multiple agent sessions on a single box/vm

[defangdevs]

As a way to keep cost low and a workaround for #4 we could allow running multiple sessions on a single box. The NixOS module already supports this, but sessions are decided at deploy time because it's 1:1 linux-user:claude-session.

[defangdevs]

Closing per Lio: the sessions feature (#60) resolved the core ask here — multiple concurrent Claude sessions on one box without the 1:1 linux-user:session coupling. The remaining variant (multiple users per box, PR #65) is being dropped: extra users on one node mainly add RAM/disk pressure. Capacity beyond one node is now tracked as scale-out in #72.