feat(work): load kube-tmux from a pinned flake input

kube-tmux is not in nixpkgs, so add the upstream repo as a flake = false
input and reference ${inputs.kube-tmux}/kube.tmux directly. The script is now
always in the store, so the previous $HOME/code/kube-tmux existence guard is
removed and the tmux status line no longer depends on a manual checkout.

Refs #50

flake.nix

@@ -60,6 +60,13 @@
       url = "github:NixOS/nixos-hardware";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    # kube-tmux: kube context/namespace for the tmux status line on the work
+    # host. Not in nixpkgs and not a flake -- pinned here as a plain source so
+    # the script is always in the store (no manual checkout). See work.nix.
+    kube-tmux = {
+      url = "github:jonmosco/kube-tmux";
+      flake = false;
+    };
   };
 
   outputs =