feat: auto-update Claude Code from nixpkgs-unstable

- Add nixpkgs-unstable input for latest Claude Code versions
- Remove duplicate home-manager config from configuration.nix (handled by flake)
- Remove fetchTarball/fetchGit calls incompatible with pure flake mode
- Add shell function to auto-update unstable on nixos-rebuild

flake.lock

@@ -74,6 +74,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1773507054,
+        "narHash": "sha256-Q8U5VXgrcxmCxPtCCJCIZkcAX3FCZwGh1GNVIXxMND0=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "e80236013dc8b77aa49ca90e7a12d86f5d8d64c9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1772822230,
@@ -94,7 +110,8 @@
       "inputs": {
         "home-manager": "home-manager",
         "kickstart-nvim": "kickstart-nvim",
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": "nixpkgs_2",
+        "nixpkgs-unstable": "nixpkgs-unstable"
       }
     },
     "systems": {