Implement hybrid package management: stable + selective unstable

- Add nixpkgs-unstable input to flake.nix
- Configure specialArgs to pass pkgs-unstable to all system configs
- Update desktop/dev.nix to use windsurf from unstable packages
- Create comprehensive documentation in README-hybrid-packages.md
- Enable selective use of cutting-edge packages while maintaining system stability

Usage: Add pkgs-unstable parameter to any .nix file and use pkgs-unstable.package-name
Example: pkgs-unstable.windsurf for latest Windsurf editor

flake.lock

@@ -52,6 +52,21 @@
         "type": "indirect"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1753694789,
+        "narHash": "sha256-cKgvtz6fKuK1Xr5LQW/zOUiAC0oSQoA9nOISB0pJZqM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "dc9637876d0dcc8c9e5e22986b857632effeb727",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-unstable",
+        "type": "indirect"
+      }
+    },
     "plasma-manager": {
       "inputs": {
         "home-manager": [
@@ -80,6 +95,7 @@
         "home-manager": "home-manager",
         "nixos-hardware": "nixos-hardware",
         "nixpkgs": "nixpkgs",
+        "nixpkgs-unstable": "nixpkgs-unstable",
         "plasma-manager": "plasma-manager"
       }
     }