logout: add --all to drain every saved login (context)

entire logout --all iterates all saved contexts, revokes each context's current session server-side against its own core (or every session on that core when combined with --everywhere), and removes each login locally — then clears the legacy keyring entry so the machine ends fully logged out. Per-context failures warn but never abort the sweep; local removal always proceeds. Adds auth.RemoveContext (named sibling of RemoveCurrentContext, sharing a keychain-cleanup helper). Also fixes the stale --all reference in the logout help left by the --all→--everywhere rename.