Scope in-file comments to the current file state (task 0001)

Address PR review feedback: 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 in task 0002" / "arrive in later
  tasks" 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.

lib/default.nix

@@ -15,8 +15,8 @@ let
 
   # --- Auto-loader ---------------------------------------------------------
   # Recursively collect every `.nix` file under `dir`, returned as a flat list
-  # of paths suitable for a module `imports`. No null-placeholder traversal
-  # hack: a directory recurses, a `.nix` file is taken, anything else is skipped.
+  # of paths suitable for a module `imports`: a directory recurses, a `.nix`
+  # file is taken, anything else is skipped.
   collectNixFiles =
     dir:
     flatten (