Rename the internal olliesrv/session package to olliesrv/mgr to resolve
the naming conflict with ollie/session (the core session package).
This allows importing ollie/session without an alias — the temporary
`agent "ollie/session"` scaffolding is removed. All references now
use the natural package name: session.X for core types, mgr.X for the
9P session manager.
Levi Neely