Starting to use Opus 4.5 I'm reduces instrutions in claude.md and just ask claude to look in the codebase to understand the patterns already in use. Going from prompts/docs to instead having code being the "truth". Show don't tell. I've found this patterns has made a huge leap with Opus 4.5.

The Ash framework takes the approach you describe.

From the docs (https://hexdocs.pm/ash/what-is-ash.html):

"Model your application's behavior first, as data, and derive everything else automatically. Ash resources center around actions that represent domain logic."

I feel like I've been doing this since Sonnet 3.5 or Sonnet 4. I'll clone projects/modules/whatever into the working directory and tell claude to check it out. Voila, now it knows your standards and conventions.