Mirrors the pattern of /permission: bare command shows the current
value plus a help line; with an argument (auto/local/cloud) it calls
Router.SetPreferPolicy and emits a system message. Session-only — does
not write back to config.toml, matching /permission and Ctrl+X
incognito-toggle conventions.
Tab completion on the value via routerPreferModes alongside the
existing permissionModes pattern. Help text updated. Status-bar
indicator deferred (separate concern if it turns out to be wanted).
vikingowl
6c5e969217