Add nix-darwin flake with personal and work hosts

Add nix-darwin flake with personal and work hosts Scaffolds the flake this repo will be driven from, with two host configurations over a shared module so the work machine does not force a refactor when it arrives. Nothing is applied yet: both hosts evaluate and dry-build, but declare no packages, defaults, or dotfiles. nix.enable is off because Determinate Nix owns the daemon and nix.conf; leaving it on makes nix-darwin contend for both on every activation. system.primaryUser is required by current nix-darwin for user-scoped options to resolve. nix-darwin and home-manager track master rather than a release branch: their release branches pair with nixpkgs releases, and this tracks nixpkgs-unstable because 26.05-darwin is a major version behind on helm and does not carry herdr or markitdown at all. Verified with `nix flake check --no-build` and a dry-build of both hosts.