Add home-manager module for launchd integration
Adds a home-manager module that allows users to enable the app as a
launchd user agent via `programs.jj-home-manager-status.enable`.
Changes:
- Move nix files under `nix/` directory for better organization
- Export `homeManagerModules.default` and `homeManagerModules.jj-home-manager-status`
- Module configures launchd agent with RunAtLoad and KeepAlive
- Add `package` option for flexibility (defaults to pkgs.home-manager-status)
Usage in consumer flake:
```nix
{
imports = [ jj-home-manager-status.homeManagerModules.default ];
programs.jj-home-manager-status = {
enable = true;
package = jj-home-manager-status.packages.\${pkgs.system}.default;
};
}
```
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