chore: post-refactor cleanups (#50-#53) (#54)

## Summary

Follow-up cleanups from the post-refactor audit (issues #50–#53). All behaviour-preserving except the work-host changes (kube-tmux + Lens removal).

## Changes

- **#51** `refactor(ssh)` — move `services.openssh.enable` + `firewall.allowedTCPPorts = [ 22 ]` into `modules/ssh.nix`; drop the duplicated lines from T400, MacPro31, RPi5.
- **#50** `fix/feat(work)` — load kube-tmux from a pinned `flake = false` input (it is not in nixpkgs) and reference `${inputs.kube-tmux}/kube.tmux` directly, so the status line no longer depends on a manual `$HOME/code/kube-tmux` checkout. (Supersedes the interim file-existence guard.)
- **#52** `chore` — gitignore the untracked `tf-inspect/` scratch project.
- **#53** `chore` — remove the unused Lens package entirely (`pkgs.lens` + its unfree entry; `unfreePackages` is now just `claude-code`), fix the `nil`→`nil_ls` LSP doc, remove the redundant `.editorconfig` block, name the RPi5 Docker subnet in a `let` binding.

## Deferred (from #53, noted in the commit)

- `.gitignore` firmware entry — documented behaviour, low value, left as-is.
- Per-eval `nixpkgs-unstable` overlay import — inherently per-system; no clean single-import hoist.

## Verification

- `nix flake check` passes (treefmt, deadnix, statix, pre-commit, all hosts + Darwin + homeConfigurations).
- Derivation-path diff vs `main`: `lyrathorpe-mbp`, `lyrathorpe-t400`, `lyrathorpe-macpro31`, `lyrathorpe-rpi5` are byte-identical (confirms #51 and the subnet `let` binding change nothing). Only `emmathorpe-edaas` differs — the kube-tmux input (#50) and the Lens removal (#53).

Closes #50, #51, #52, #53.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #54

.gitignore

@@ -2,3 +2,6 @@ modules/firmware/*
 
 # vim swap files
 *.swp
+
+# Local scratch project, not part of this flake.
+tf-inspect/