Add OpenTofu host tools for AWS EC2

[jimpudar]

Summary

• Add opentofu to the Nix hostTools flake output for macOS host usage.
• Switch AWS EC2 host-tool resolution to prefer tofu while still honoring ROOTCELL_TERRAFORM for a user-provided Terraform binary.
• Update README and provider docs to describe OpenTofu as the default and keep Terraform as an override path.
• Add unit coverage for the new default/override behavior and update the AWS EC2 integration preflight check.

Testing

• bun run typecheck
• bun run lint
• bun run test src/rootcell/rootcell.test.ts
• nix eval --raw .#packages.aarch64-darwin.hostTools.name