docs: simplify code comments, keep the detail in the README

The registry / multi-user / portable-home rationale lives in the README
(Users, Hosts, Portable home); the corresponding code comments are reduced
to one-liners that point there. No functional change (host derivations are
unchanged).

flake.nix

@@ -97,10 +97,7 @@
           "lens-desktop"
         ];
 
-        # Identity registry: who each user is (name, email, keys, groups), keyed
-        # by username. Threaded into the system layer as the `userRegistry`
-        # specialArg and into each user's home config as the `identity` module
-        # arg. See users/registry.nix, modules/users.nix, home/git.nix.
+        # Per-user identity, keyed by username. See README "Users".
         userRegistry = import ./users/registry.nix;
 
         # nixpkgs + nix-daemon settings shared by NixOS and Darwin hosts.
@@ -132,14 +129,8 @@
           }
         ];
 
-        # mkHost :: { system, modules, users, portable } -> nixosSystem
-        # Builds one machine by appending its host-specific modules to the shared
-        # baseModules. `users` is an attrset keyed by username; each value carries
-        # that user's home-module list and optional per-host-user system bits
-        # (e.g. linger). Per-user identity is injected into each home config via
-        # the `identity` module arg (extraSpecialArgs is per-host, so it cannot
-        # carry per-user data); the system layer reads the global `userRegistry`
-        # restricted to this host's `hostUsers` set.
+        # Build one NixOS host. `users` is an attrset keyed by username (home
+        # modules + optional per-user system bits). See README "Users".
         mkHost =
           {
             system,
@@ -190,11 +181,8 @@
           }
         ];
 
-        # mkDarwinHost :: { system, username, modules, homeModules } -> darwinSystem
-        # Darwin counterpart of mkHost. macOS already owns the login user, so we
-        # only attach the platform and home-manager; no NixOS user module here.
-        # Stays single-user (macOS owns the account); identity is still sourced
-        # from the registry so the shared home modules behave as on NixOS.
+        # Darwin counterpart of mkHost: single-user (macOS owns the account),
+        # identity still from the registry. See README "Users".
         mkDarwinHost =
           {
             system,
@@ -223,12 +211,8 @@
               ];
           };
 
-        # Host table — declarative registry of every machine. To add a host:
-        # give it a name, its `system`, its `users` set (each user's home-module
-        # list, plus optional per-host-user bits like linger), and the system
-        # `modules`. mapAttrs below turns each entry into a nixosConfiguration of
-        # the same name. Per-user home configs compose ./home (the shared bundle)
-        # with any per-user modules (e.g. ./users/emmathorpe/work.nix).
+        # Host table — one entry per machine, realised into a nixosConfiguration
+        # of the same name below. See README "Hosts" / "Users".
         hosts = {
           lyrathorpe-mbp = {
             system = "aarch64-linux";
@@ -418,14 +402,8 @@
         flake.nixosConfigurations = lib.mapAttrs (_name: mkHost) hosts;
         flake.darwinConfigurations = lib.mapAttrs (_name: mkDarwinHost) darwinHosts;
 
-        # Reusable home modules, exported so this config can be consumed off these
-        # hosts -- by a standalone home-manager on a non-NixOS machine, or as an
-        # input to someone else's flake. `default` is the portable bundle
-        # (shell + git + editor + claude). Consumers must supply the module args
-        # these expect: `inputs` always; `identity` (see users/registry.nix) for
-        # git/desktop; `portable` for sway. `desktop`/`sway` additionally require
-        # a NixOS host that provides the Sway/Firefox binary -- they are not
-        # standalone-portable.
+        # Reusable home modules, exported for use off these hosts. See README
+        # "Portable home" for the consumer module-arg expectations.
         flake.homeModules = {
           default = ./home;
           shell = ./home/shell.nix;
@@ -436,13 +414,8 @@
           sway = ./home/sway.nix;
         };
 
-        # Standalone home-manager configurations: the portable bundle built for a
-        # machine NOT managed by this flake (`home-manager switch --flake
-        # .#"<user>@<system>"`). Only the portable subset is exposed; the desktop
-        # suite stays NixOS-only. homeConfigurations are not per-system, so the
-        # system is encoded in the attribute name, and home.username/homeDirectory
-        # are set explicitly (the NixOS module sets them automatically; standalone
-        # does not).
+        # Standalone home-manager configs (portable bundle) for machines not
+        # managed by this flake. See README "Portable home".
         flake.homeConfigurations =
           let
             mkHome =