Nice.

For the message, how about:

Just a heads up: we're updating UCM's project naming rules to match Unison Share. The following project names will need a quick rename to keep everything running smoothly in upcoming releases:

a, a_a, éééééééééé

Just use `project.rename` and switch to one that only includes ASCII letters, digits, and hyphens; between 2–40 characters.

In a future PR we can switch project.create, project.create-empty, project.rename to only allow new names that match the regex, even while still giving the warning for legacy names.

Mainly just avoiding the phrase "will all be invalid".

@aryairani I'm happy to make changes here but I think your suggestion "The following project names will need a quick rename to keep everything running smoothly in upcoming releases" is kinda wordy and confusing to read.

Could we perhaps tease out what specifically is wrong with the phrasing "These project names will all be considered invalid in an upcoming UCM release" and then iterate from there?

@mitchellwrosen Good point about it being kind of long. How about

⚠️ 

We're updating UCM's naming rules to match Unison Share, and these project names won’t be supported much longer:

a, a_a, éééééééééé

Please project.rename them using only ASCII letters, numbers, and hyphens (2–40 characters).

Could we perhaps tease out what specifically is wrong with the current phrasing and then iterate from there

Just trying to match tone with the rest of the UCM, which aims friendlier and less 🤖 "error: invalid data" than most compiler tools. Therefore avoiding the phrase "all will be invalid".

Ok, the point about tone sounds good, I think I still have a small issue with this error message though, which is that this feels unnecessary to disclose to the user (and therefore unnecessarily wordy):

We're updating UCM's naming rules to match Unison Share

Like, yeah, we screwed up and used two different regexs for project names in two different places and now have to migrate our way out of this situation 😅.

How about just starting at the "these project names won't be supported much longer" part?

Yeah, when you put it that way about the regexes...
But I do want to include a "why" whenever practical.

We're updating UCM's project naming rules, and these names won’t be supported much longer:

This is a pretty weak why, but it's better than nothing

CI still seems to be failing, unrelated to this PR

@aryairani Ready