cli: tidy

[oliver-sanders]

Having gone through the command list while working on the rose suite-run migration proposal, propose a quick tidy of commands and command help:

1. Hide all "internal" commands by default.
2. Drop the category system for invoking commands (i.e. the task in cylc [task] message).
3. Consider dropping the category system.
4. Replace % cmd code examples with $ cmd (because bashism, but also colour formatting)
5. Remove the following commands in due course: (commented on Rationalize the CLI command set. #1249)
   • check-software - outdated and messy, setup.py ensures dependency compatibility.
   • checkpoint - Remove named checkpointing #3891
   • client - somewhat obsolete since GraphQL, remove or rename to query to match subscribe.
   • get-directory - will be made obsolete by cylc install #3889
   • ls-checkpoints - Remove named checkpointing #3891
   • register - will be made obsolete by cylc install #3889

Ping @hjoliver

[hjoliver]

Agreed on all items 👍

I'll remove the question label. Others can argue points if they want, but it all seems pretty uncontroversial to me.

[oliver-sanders]

Should we add cylc nudge to the kill list? Task ETA is now computed client-side.

[hjoliver]

Yes, kill it.