migrate: Replace yarn with pnpm package manager
- Remove yarn.lock and replace with pnpm-lock.yaml
- Update package.json scripts to use pnpm
- Configure lerna.json to use pnpm as npm client
- Add pnpm-workspace.yaml for workspace configuration
- All tests pass for @did-plc/lib package
- Build process verified and working
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Co-Authored-By: Claude <noreply@anthropic.com>