chore: minor cleanups (lens unfree name, LSP doc, editorconfig, docker subnet)

- flake.nix: drop the dead "lens" unfree entry; pkgs.lens is named
  "lens-desktop", which is the name the predicate actually matches.
- 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

@@ -93,6 +93,7 @@
         # 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

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