chore: Migrate from Yarn 1 (Classic) to Yarn 3+

[lucas-zimerman]

Context

Yarn 1 (Classic) has been in maintenance mode since 2020 and we've been hitting issues caused by its aging dependency resolution.

Most recently, during the JS SDK bump to 10.60.0 (#1303), Yarn 1 generated a corrupt yarn.lock entry for micromatch@4.0.8 — omitting its braces and picomatch dependencies entirely. This caused CI to fail with Cannot find module 'braces' and required manually patching the lockfile plus adding a braces resolution workaround in package.json.

Proposal

Migrate to Yarn 3+ (Berry) using nodeLinker: node-modules mode for maximum compatibility. This would give us:

• Reliable lockfile generation and dependency hoisting
• Better performance and caching
• Active maintenance and security updates
• Stricter dependency resolution that catches issues like the above at install time

Scope

• Run corepack enable && yarn set version stable
• Configure .yarnrc.yml (likely nodeLinker: node-modules for compatibility)
• Update .gitignore for Yarn Berry artifacts (.yarn/cache, .yarn/releases, etc.)
• Update CI workflows for Yarn 3 commands
• Verify sample apps and .yalc setup still work
• Remove braces resolution workaround from package.json once lockfile is reliable
• Update contributing docs if any

[linear-code]

CAPA-94