chore: contacts & events commands should use cobra for flag constraints

[notnmeyer]

i homerolled some flag constraints for events and contacts originally. found out later that cobra has functions that do this. this also make it easier to programmatically inspect a command (like for producing agent help output).

for example,

➜ task -- events send blah

   ERROR

  At least one of the flags in the group [email user-id] is required.