Implements supports_umbrella? callback

[Wigny]

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Tested in a private umbrella app by running mix igniter.install usage_rules@github:wigny/usage_rules@support-umbrella and then mix usage_rules.sync AGENTS.md --all.

Closes #32.

[Wigny]

Please let me know if there are any unit tests that we could add for this callback, in case that is not already covered by Igniter itself.

[zachdaniel]

🚀 Thank you for your contribution! 🚀

[nathanalderson]

I'm trying to use this in an umbrella app and struggling a bit. I have installed from tip to get this change, but when I run from the root of the project, it only detects its own usage rules and doesn't pick up dependencies used by my subprojects (such as Phoenix used by apps/web).

» mix usage_rules.sync --list                                 

Notices: 

* usage_rules
    usage_rules:otp - has sub-rule
    usage_rules:elixir - has sub-rule
    usage_rules - has usage rules

Notices were printed above. Please read them all before continuing!

Is this expected?

[zachdaniel]

🤔 Are you making multiple rules files?

[nathanalderson]

No. Just followed the most basic setup instructions. Added it as a dependency to my root umbrella project, then ran mix usage_rules.sync AGENTS.md --all. I do have usage rule files in my deps folder:

» ls deps/phoenix/usage-rules  
ecto.md  elixir.md  html.md  liveview.md  phoenix.md

But just to be clear, phoenix isn't a direct dependency of the root project, only of one of the subprojects.

[zachdaniel]

🤔 I think all of the deps should be available in the root though 🤔 So it should work? I'd have to investigate and unfortunately I'm a bit busy at the moment. The task itself isn't too complex, happy to guide someone through the process of working it out.

[nathanalderson]

See #37