style: scope in-file comments to the current file state

In-file comments should describe only what the file currently is, not task
numbers, external tooling, or past/future states.

- hosts/neogaia: drop the "replaced by disko" / "arrive later" framing;
  describe the filesystems and hardware profile as the placeholder values
  they are.
- lib: drop the "no null-placeholder traversal hack" comparison to a prior
  implementation.
- system: drop "no impure environment lookup" from the user.name description
  and the sops/post-boot roadmap from the user comment.

hosts/neogaia/hardware-configuration.nix

@@ -1,7 +1,6 @@
 { lib, modulesPath, ... }:
-# Placeholder hardware profile — regenerated on the target machine at install
-# time (`nixos-generate-config` / `disko-install`). Carries only enough for the
-# toplevel to evaluate: the host platform and the XPS 13's initrd modules.
+# Placeholder hardware profile for the XPS 13: the host platform and the initrd
+# modules the machine needs to evaluate and boot.
 {
   imports = [ (modulesPath + "/installer/scan/not-detected.nix") ];