Add Pest v4 guidelines

[ashleyhindle]

• Linear: https://linear.app/laravel/issue/AI-77/add-pest-v4-guidelines

What & Why

Pest 4 will be out soon and we want people using Boost to get a headstart.

How I tested

1. Installed Pest 4 into a test project
2. Ran boost:install
3. Used claude to ask:
   • "how do pest smoke tests work? What's the code for them? Dont' add one, just tell me" -> search-docs "pest smoke testing" where it showed me the exact code needed
   • "how do i shard my tests?"
   • "Add a browser test for the homepage, click on register, then register with faker() details, and make sure we get redirected to the dashboard"

Notes

• Should we nudge browser tests to live in tests/Browser/ or no?

Demo

Using Pest docs to write a browser test, then running Pint to make sure the code is okay ❤️