Allow guideline overriding

[ashleyhindle]

• Linear: https://linear.app/laravel/issue/AI-96/allow-guideline-exclusion-and-overriding

What & Why

Some people only have Inertia in part of their project, they don’t want Boost's guidelines recommending to create every new page with Inertia.

They should be able to put a file in .ai/guidelines/inertia-laravel/core.blade.php that we use instead of Boost's.

This opens up the door for a lot of customisation.

• Update README.md
• If overridden file exists, use that instead of Boost's
  • even/especially if it's empty.

How I tested

1. Ran the tests
2. Ran boost:install in a demo project with custom guidelines and verified the guideline files include the overridden files, not the Boost files.

This makes sure custom guidelines still work, and overriding Boost guidelines works.

❯ tree .ai
.ai
└── guidelines
    ├── enforce-tests.blade.php
    ├── livewire
    │   └── 3
    │       └── core.blade.php
    └── tdd.blade.php

Notes/thoughts

• There's a future task to enable publishing guidelines to modify. And possibly a future task to allow excluding guidelines in the config.
• I think we should soon move Boost guidelines into .ai/guidelines/ in the package too, to match the custom guidelines, so they're the same.

[pushpak1300]

LGTM ! 🚀
Tested locally with custom guidelines. works as expected.
Thanks.