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.lock

@@ -191,6 +191,22 @@
         "type": "github"
       }
     },
+    "kube-tmux": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1779714285,
+        "narHash": "sha256-l1wjg2ReWKCI7h/K11vvX2ykYTs/mVD+tfz/mQsjn/E=",
+        "owner": "jonmosco",
+        "repo": "kube-tmux",
+        "rev": "8b7e1d127c16b6dc87ff5743f4d775b245198b69",
+        "type": "github"
+      },
+      "original": {
+        "owner": "jonmosco",
+        "repo": "kube-tmux",
+        "type": "github"
+      }
+    },
     "nix-darwin": {
       "inputs": {
         "nixpkgs": [
@@ -373,6 +389,7 @@
         "flake-parts": "flake-parts",
         "git-hooks": "git-hooks",
         "home-manager": "home-manager",
+        "kube-tmux": "kube-tmux",
         "nix-darwin": "nix-darwin",
         "nix-homebrew": "nix-homebrew",
         "nix-index-database": "nix-index-database",