PTY per-unit resource limits

Trap: aggregate opencode-pty.slice limits and a global session count look sufficient, but sibling transient PTY cgroups share the slice, so one session can consume its entire memory/task allocation and starve others; CPU, I/O, storage, and network may also remain unbounded. Fix: retain aggregate limits and atomic capacity reservation, while assigning every transient unit independent memory/task ceilings plus appropriate CPU, I/O, storage, and network limits.