style: 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/hardware-configuration.nix

@@ -1,6 +1,5 @@
 { lib, modulesPath, ... }:
-# Placeholder hardware profile for the XPS 13: the host platform and the initrd
-# modules the machine needs to evaluate and boot.
+# Placeholder: regenerate with nixos-generate-config on the target machine.
 {
   imports = [ (modulesPath + "/installer/scan/not-detected.nix") ];