chore: minor cleanups (remove unused lens, LSP doc, editorconfig, docker subnet)

- Remove the unused Lens package entirely: drop pkgs.lens from the work host
  and its unfree entry from flake.nix. Nothing else needed it, so
  unfreePackages is now just claude-code. (pkgs.lens has pname
  "lens-desktop", which was the string the unfree predicate matched.)
- home/README.md: the Nix LSP is nil_ls, not nil.
- .editorconfig: remove the rule block that duplicated the [*] defaults.
- hosts/RPi5/docker.nix: name the trusted Docker subnet in a let binding
  rather than repeating the literal CIDR.

Deferred from the audit bundle: the .gitignore firmware entry (documented,
low value) and the per-eval nixpkgs-unstable overlay import (inherently
per-system, no clean hoist).

Closes #53

flake.nix

@@ -93,7 +93,6 @@
         # Unfree packages permitted to be built (replaces blanket allowUnfree).
         unfreePackages = [
           "claude-code"
-          "lens-desktop" # the name of pkgs.lens (used on the work host)
         ];
 
         # Per-user identity, keyed by username. See README "Users".

users/emmathorpe/work.nix

@@ -24,7 +24,6 @@
     pkgs.powershell
     pkgs.nuget
     pkgs.gedit
-    pkgs.lens
     pkgs.python3
     pkgs.gnumake
     pkgs.gcc