Give the Host-side placement a `limits` field — `memory`, `cpu`, and
`tasksMax` — that caps the guest's `container@<name>.service` unit via
`MemoryMax`, `CPUQuota`, and `TasksMax`, uncapped by default so an unset
limit contributes no key and systemd keeps its default.
Add an `autoStart` placement option, on by default, driving the
container's boot-start so a Host can define a guest without starting it
at boot.
Give neogaia's skeleton guest modest demonstrative caps.
