Cobra keeps completion callbacks in a package-global map keyed by
*pflag.Flag with no removal path, so registrations made during Build()
outlive the command itself. Route all seven call sites through
cmdutil.RegisterFlagCompletion and enable registration only when the
invocation actually serves a __complete request.
Measured over 30 dropped Builds: ~202 KB / 2180 retained objects per
Build before, ~0 after.
Change-Id: I734d598a4c91a92c33b02e0f292f640cc0e224c6