nix: install gmcli on NixOS and nix-darwin

[colonelpanic8]

Summary

• add the gmcli flake as an input to both the NixOS and nix-darwin flakes
• make the gmcli package follow each configuration's existing nixpkgs input
• install gmcli through the shared essential system package list
• lock both configurations to the tested gmcli revision

Why

gmcli now publishes a cross-platform Nix flake. Installing it from the shared essential package module makes the CLI available by default on both NixOS and nix-darwin without duplicating its build definition.

This PR currently follows colonelpanic8/gmcli:agent/json-archive-export, which is the head of fdsouvenir/gmcli#2. It can be switched to upstream after that PR merges.

Validation

• formatted all changed Nix files with Alejandra
• evaluated mac-demarco-mini and confirmed gmcli is in environment.systemPackages
• built the locked aarch64-darwin gmcli package with its Go tests enabled
• evaluated gmcli derivations for both x86_64-linux and aarch64-linux

Full NixOS host evaluation is not available from this Apple Silicon host because the existing NixOS flake realizes Linux-only patch derivations during evaluation.