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/disk.nix

@@ -1,13 +1,7 @@
 { ... }:
-# neogaia's disk layout, declared with disko and interpreted by the disko module
-# the host-builder wires in. This is a per-Host concern: another Host declares a
-# different `disko.devices` (or none, preserving an existing pool by importing it).
-#
-# One NVMe disk, GPT: an EFI system partition for systemd-boot, and a LUKS
-# container holding a btrfs filesystem with subvolumes. Swap is zram (RAM-backed),
-# so there is deliberately no on-disk swap partition. disko derives the matching
-# `fileSystems.*` and `boot.initrd.luks.devices.*` from this, so a normal boot
-# prompts for the passphrase in the initrd and unlocks the encrypted root.
+# neogaia's disk layout for disko: one NVMe disk, GPT, with an EFI system
+# partition and a LUKS container holding btrfs subvolumes. No swap partition;
+# swap is zram. disko derives `fileSystems` and `boot.initrd.luks.devices` from this.
 {
   disko.devices.disk.main = {
     type = "disk";