#38 document entity addressing: issue <N> | task|pr <uuid> | milestone|label <name> | wiki <path>

SOURCE: swarm friction #4 (verified: issues by number, milestones by name, tasks by UUID, nothing documents which is which). FIX: ONE authoritative line in spec [global rules] AND a notes line in the relevant -h: "address: issue <N> | task|pr <uuid> | milestone|label <name> | wiki <path>". DONE: addressing is documented in exactly one place, visible from help.