Split README into generic rootcell docs and macOS+Lima provider guide

[jimpudar]

Summary

• Reworked the top-level README to describe rootcell generically across lima and aws-ec2 instead of being Lima-specific.
• Added a new macOS + Lima provider README with provider setup, host requirements, Lima layout, user-v2 network behavior, image details, and architecture notes.
• Corrected stale instance defaults in .env.defaults and aligned the sample IPs with the current allocator.
• Added a unit test that locks in the new Lima provider README coverage.

Testing

• bun run test -- src/rootcell/rootcell.test.ts
• Scoped whitespace/diff check passed on the edited files.