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

hosts/RPi5/docker.nix

@@ -8,6 +8,10 @@
 # secure upgrade path is mutual TLS on 2376 (--tlsverify with client certs);
 # that needs out-of-band cert provisioning and is intentionally not wired here.
 { ... }:
+let
+  # LAN allowed to reach the unauthenticated Docker TCP socket (see SECURITY above).
+  trustedSubnet = "10.187.1.0/24";
+in
 {
   virtualisation.docker.enable = true;
 
@@ -29,6 +33,6 @@
   # CIDR to match the LAN that should reach the Docker API.
   networking.nftables.enable = true;
   networking.firewall.extraInputRules = ''
-    ip saddr 10.187.1.0/24 tcp dport 2375 accept
+    ip saddr ${trustedSubnet} tcp dport 2375 accept
   '';
 }