feat(auth): streamline SOPS authentication and network configuration

- Replace age-based key decryption with automatic SSH host key support
- Simplify SSH configuration with consolidated host entries and enhanced security settings
- Upgrade home-manager to himmelblau for Microsoft Entra ID authentication
- Remove legacy agenix inputs and consolidate secrets to essential services only
- Add Cisco Secure Client overlay for enterprise VPN capabilities
- Update secrets.yaml to minimal configuration with gitea/github/hetzner services
- Expand system packages with tun module for VPN connectivity
- Add work directory setup for user development environment

flake.nix

@@ -82,14 +82,6 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
 
     # rest of inputs, alphabetical order
-    agenix = {
-      url = "github:ryantm/agenix";
-      inputs = {
-        nixpkgs.follows = "nixpkgs";
-        systems.follows = "systems";
-      };
-    };
-
     import-tree.url = "github:vic/import-tree";
 
     zen-browser = {
@@ -102,6 +94,11 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    himmelblau = {
+      url = "github:himmelblau-idm/himmelblau/main";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     nix-index-db = {
       url = "github:Mic92/nix-index-database";
       inputs.nixpkgs.follows = "nixpkgs";
@@ -127,4 +124,4 @@
       inputs.nixpkgs.follows = "nixos-24-05";
     };
   };
-}
+}