Remove host-side Nix fallback from runtime setup

[jimpudar]

Summary

• Stop ./rootcell from invoking host-side nix build to locate vfkit, zstd, or python3
• Resolve host tools from env vars or PATH, with explicit install guidance when missing
• Add a Nix hostTools package for users who still want a Nix-managed macOS setup
• Rewrite the README to separate Homebrew and Nix host setup paths and clarify that Nix is only required for image builds and guest provisioning

Testing

• Added unit coverage for host-tool resolution and for preventing runtime host-side Nix fallbacks
• Ran bun test, bun run typecheck, bun run lint, and nix eval against the new host-tools output

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