Remove root CLAUDE.md in favor of AGENTS.md (#58600)

Summary:
Removes the root `CLAUDE.md`, which only contained `AGENTS.md`. As of [Claude Code v2.1.277](https://github.com/anthropics/claude-code/releases/tag/v2.1.277), Claude Code reads `AGENTS.md` directly in a project with no `CLAUDE.md`, so the import shim is redundant. Keeping a single instruction file avoids the two drifting apart.

Scope: only the root `CLAUDE.md`. `AGENTS.md` and `packages/react-native-compatibility-check/AGENTS.md` are unchanged.

## Changelog:

[INTERNAL] - Remove redundant root CLAUDE.md in favor of AGENTS.md

Pull Request resolved: #58600

Test Plan: `git grep CLAUDE.md` returns no references to the file anywhere in the repo. No code or tests are touched.

Reviewed By: christophpurrer

Differential Revision: D120809883

Pulled By: fabriziocucci

fbshipit-source-id: c699b3c7bba9d26869dfab83cd5f8fa1b881aef3

Author: jarrensj