feat: add usergroups write commands with a --yes confirmation gate (#156)

Adds create, update, add, remove, enable and disable to the usergroups group, completing the read/write split opened in #148.

add and remove are read-modify-write against usergroups.users.update, which replaces the whole member list: they read the current membership, apply the change, skip a no-op, and refuse to empty the last member. Every write confirms first, and refuses with a non-zero exit when stdin is not a TTY and --yes is absent.

Closes #139