I'd be very much in favor and I'd be happy to contribute changes for this. I've added type hints in my local fork to assist in development. Especially for core utility functions like idx and id which otherwise completely hide type information.

The code to select which tests to run by default is in ArcanistWorkflow::selectPathsForWorkflow().

Suggested meme for closing: "yesyes"

How an extension can have their unit tests then? 🤔

I wonder if xhpast can be ditched. Eg:
For linters, use something off-the-shelf such as phpcs (https://github.com/squizlabs/PHP_CodeSniffer)
For diviner parsing of PHP code, I look at the class documentation (eg https://we.phorge.it/book/libphutil/class/ArcanistConduitCallFuture/) and question as to whether this provides anyone with any benefit.

I've created R10 PhorgeIt Extension for this, with a little bit of content. I'll load it to the server shortly.