feat(INFRA-2931): cache yarn packages

[Qbandev]

What

Adds yarn offline mirror caching to speed up CI builds by 5-10x while maintaining security requirements.

Why

• CI builds download all packages on every run (3-5 min per job)
• NPM registry failures cause due to transient network error
• High GitHub Actions minutes usage costs
• Cannot use actions/cache directly due to security (see Update workflows to use MetaMask/action-checkout-and-setup core#5474)

How

• Caches yarn's offline mirror (.yarn-offline-mirror/) containing package tarballs
• Uses --prefer-offline when cache is available (30-60s vs 3-5min installs)
• Only enabled in low-risk environments for security
• Fully backward compatible - no workflow changes needed

Checklist

• Version bumped to 1.3.0
• CHANGELOG.md updated
• README.md documentation added
• Backward compatible
• Security requirements met

Resolves: INFRA-2931