Trim verbose comments to concise, self-contained notes

Cut restated "what", domain-glossary framing, cross-file consumption
narration, and against-alternative justification from in-file comments;
keep only non-obvious "why" and load-bearing pointers. Drop the
`generateCompletions` line (a no-op restatement of the upstream default)
and its comment.

hosts/neogaia/default.nix

@@ -1,8 +1,6 @@
 { pkgs, ... }:
 # neogaia — Dell XPS 13 9380 laptop.
-#
-# The disk layout lives in ./disk.nix (disko); the resulting `fileSystems` are
-# derived from it, so none are declared by hand here.
+# Disk layout is in ./disk.nix; `fileSystems` are derived from it, none declared here.
 {
   imports = [
     ./hardware-configuration.nix
@@ -11,44 +9,34 @@
 
   system.stateVersion = "26.05";
 
-  # systemd-boot on the EFI system partition disko creates. The initrd prompts
-  # for the LUKS passphrase (disko wires up boot.initrd.luks.devices), so a
-  # normal boot unlocks the encrypted root.
+  # systemd-boot on the EFI system partition.
   boot.loader.systemd-boot.enable = true;
   boot.loader.efi.canTouchEfiVariables = true;
 
-  # neogaia runs the CachyOS kernel, selected per-Host via boot.kernelPackages.
   boot.kernelPackages = pkgs.linuxPackages_cachyos;
 
-  # Intel CPU microcode updates for the XPS 13's Core i7-8565U.
   hardware.cpu.intel.updateMicrocode = true;
 
-  # Redistributable firmware — carries the ath10k blobs the QCA6174 wifi needs.
+  # Redistributable firmware for the QCA6174 wifi (ath10k blobs).
   hardware.enableRedistributableFirmware = true;
 
-  # Swap is RAM-backed zram rather than an on-disk partition.
+  # RAM-backed swap; no on-disk swap partition.
   zramSwap.enable = true;
 
-  # NetworkManager drives the wifi so it can be joined from the console.
+  # So wifi can be joined from the console.
   networking.networkmanager.enable = true;
 
-  # An SSH daemon so the rest of the setup can be driven over the network.
+  # So setup can be driven over the network.
   services.openssh.enable = true;
 
   # fish as the login shell.
   modules.fish.enable = true;
   modules.fish.defaultShell = true;
 
-  # tmux as the terminal multiplexer.
   modules.tmux.enable = true;
-
-  # Neovim, configured declaratively via nixvim.
   modules.nvim.enable = true;
-
-  # Claude Code, Anthropic's CLI, installed via home-manager.
   modules.claude-code.enable = true;
 
-  # Locale preferences for the base system.
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";
   console.keyMap = "us";