(feat): Create homebrew install for CLI and rename to nixo

[HashWarlock]

The current install path emphasizes Nix builds from source:
nix build github:HashWarlock/nixosandbox
Run binary from ./result/bin/nixosandbox

The goal is to make install and usage easier for mainstream users and agent runtimes:

1. Install through Homebrew without requiring users to build the whole project with Nix.
2. Use nixo as the primary CLI command.
3. Keep nixosandbox as a compatibility alias.
4. Provide a cross-agent skill using the Agent Skills standard so any compliant runtime can use the CLI reliably.

[HashWarlock]

implemented in #6