UCM should warn the user when they push to share for the first time for a project with a non-main branch

[hojberg]

Take this scenario:

.> project.create @hojberg/someProject
@hojberg/someProject> branch /myFeatureBranch
 ... work work ..
@hojberg/someProject> push

Share will end up with a project without a default branch (since myFeatureBranch was pushed, but main never was), and many features will not be enabled.
UCM should display a warning and clear instructions for resolution.

Something like (very rough sketch, included to convey the idea):

I just created @hojberg/someProject with the branch of `myFeatureBranch` on Unison Share.
This new Share project is missing a default branch (it now only has `myFeatureBranch`),
and some features are disabled. Create a default branch by merging your work into `main`
and pushing that when ready.

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Related: #4852