sysinfo's refresh_all never forgets a process

Linux companion agent reports CPU and memory using the Rust sysinfo crate. After 17 days one host’s agent was holding 2.33 GB, growing by about 130 MB a day.

The agent called refresh_all() on every poll. In sysinfo 0.32.1 that refreshes the process list as well, and adds every process it sees, but does not remove the ones that have exited. On a machine that starts many short-lived processes the list only grows.

The agent needs no process information, so it now refreshes CPU and memory specifically. A regression test holds memory flat across many polls.