control: positional-argument refusals should print the usage line #215

The output rules (#183, Users wiki "Output rules") say a bad invocation prints usage: and the registered usage. c.usage() does that for the call sites that had a usage literal, but the shared argument helpers (resolveRepo, issueRef, mrRef, releaseRef and friends) refuse a missing positional with expected <owner/name> <number> and no usage line:

$ gitbay issue show
expected <owner/name> <number>

Have those helpers append the registered usage (c.Cmd.Usage) on a second line, or route them through c.usage() with the specific "expected" text first, so every exit-2 path names the usage.

Ref #183