Better way to show/hide advanced gptel options in the menu

[karthink]

Presently, some advanced gptel options can be shown in its transient menu by setting the variable gptel-expert-commands. These include dry-run commands, the model temperature and more. They are hidden by default to avoid confusing or overwhelming new users.

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The right way to handle item visibility in transient menus is to assign them to different transient levels, which you can choose or set persistently by pressing C-x l in a transient menu. (To show all levels temporarily you can use C-x a.) This is how Magit handles "hidden" commands.

I tried that, but users were quite confused by the instructions. Saying "Run (setq gptel-expert-commands t)" is simpler to understand, since everyone knows what setq does and the option name speaks for itself. Saying "In the transient menu, press C-x l and assign the dry-run commands to level 4 or under" is confusing when you're already trying to debug a problem with gptel. So I reverted this change.

But the gptel-expert-commands method isn't great either. I just left it as a defvar for later, hoping to figure out a better system for showing advanced commands in the menu.

So here we are. If we make it a defcustom that pretty much sets it in stone. Switching to another method later will annoy users who have this set in their configurations. As a defvar the setting is somewhat "internal". I'm not sure what to do.

Originally posted by @karthink in #435 (comment)