docs: clarify Yarn 4 (Berry) and Corepack requirements for local development

Problem

The Fuselage repository uses Yarn 4 (Berry), but this is not clearly stated in the README.

New contributors, especially on Windows, often attempt to run:

• npm install, or
• Yarn Classic (v1)

This leads to confusing errors, failed workspace builds, or broken installs due to the presence of .yarnrc.yml, .yarn/, and Yarn 4–specific behavior.

Why this matters

A failed local setup is often the first interaction a contributor has with the codebase.
Unclear tooling requirements can cause contributors to give up before making their first contribution.

Proposed improvement

• Explicitly state that the repository uses Yarn 4 (Berry).
• Recommend Corepack as the supported way to install and manage Yarn.
• Add a short Windows-specific note about long path issues.
• Clarify that peer dependency warnings during yarn install are often expected in this monorepo.

This would significantly improve contributor onboarding without changing any runtime or production code.

Start with the README and review the existing .yarnrc.yml and .yarn/ setup to ensure the guidance matches the repository. Update the contributor setup instructions with Yarn 4, Corepack, Windows long-path guidance, and expected peer dependency warnings; done means a new contributor can choose the supported install path without confusing Yarn or npm errors.