chore: migrate to Corepack-managed Yarn 4

[tomcasaburi]

Summary

• pin the repo to Yarn 4 via Corepack
• remove reliance on global Yarn Classic
• update local setup, CI, and docs for the new workflow

Testing

• yarn install --immutable
• yarn build
• yarn type-check
• yarn lint
• yarn prettier
• yarn test

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Note

Medium Risk
Moderate risk because it changes dependency installation semantics in CI and local dev (Corepack + yarn install --immutable), which can cause workflow breakage if the pinned Yarn/lockfile expectations differ across environments.

Overview
Pins the repo to Corepack-managed Yarn 4 by adding packageManager: "yarn@4.13.0", .yarnrc.yml, and .nvmrc, and updates contributor guidance to run nvm + corepack enable.

Updates CI to enable Corepack and switch installs from Yarn Classic (--frozen-lockfile) to Yarn 4’s yarn install --immutable, and expands workflow triggers to include .yarnrc.yml. Adds .yarn/* ignore rules (with Yarn-managed subfolders allowlisted) and lightly aligns hook/docs wording around “Yarn install”.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 89e94e51-5500-46dc-b80e-d95468d1a247

📥 Commits

Reviewing files that changed from the base of the PR and between 206dc0a and a3c2184.

⛔ Files ignored due to path filters (1)

• yarn.lock is excluded by !**/yarn.lock, !**/*.lock

📒 Files selected for processing (12)

• .codex/hooks/yarn-install.sh
• .cursor/hooks/yarn-install.sh
• .github/workflows/CI.yml
• .gitignore
• .nvmrc
• .yarnrc.yml
• AGENTS.md
• README.md
• docs/agent-playbooks/hooks-setup.md
• docs/mock-content.md
• docs/testing.md
• package.json

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/yarn4-corepack

📝 Coding Plan

• Generate coding plan for human review comments

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

CI dependency caching removed without replacement

Medium Severity

The cache: 'yarn' option was removed from actions/setup-node in both the checks and e2e jobs without adding any alternative Yarn dependency caching. Every CI run now does a fresh yarn install with no cache, which can add several minutes per run. The corepack enable step runs after actions/setup-node, but moving it before setup-node and restoring cache: 'yarn' would fix the compatibility issue that likely motivated the removal.

Additional Locations (1)

• .github/workflows/CI.yml#L91-L96