feat: Implement two-key SSH management strategy

- Add modules/security/ssh-keys.nix for centralized SSH key management
- Generate role-specific SSH keys with geir@geokkjer.eu email:
  - Admin key (geir@geokkjer.eu-admin) for sma user server access
  - Development key (geir@geokkjer.eu-dev) for geir user and git services
- Update SSH client config with role-based host patterns
- Configure users/geir.nix and users/sma.nix with appropriate key access
- Add SSH key setup to both machine configurations
- Create scripts/setup-ssh-keys.sh for key generation automation
- Update plan.md with completed SSH security implementation

Security benefits:
- Principle of least privilege (separate admin vs dev access)
- Limited blast radius if keys are compromised
- Clear usage patterns: ssh admin-sleeper vs ssh geir@sleeper-service.home
- Maintains compatibility with existing services during transition

modules/users/geir.nix

@@ -21,6 +21,12 @@
     
     shell = pkgs.zsh;
     
+    # SSH access with development keys
+    openssh.authorizedKeys.keys = config.security.ssh-keys.development or [
+      # Fallback to current key during transition
+      "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHeOvTHIw+hZOAiWkIrz9t11UeGwxAMx7jN/1IIdgq7O geokkjer@gmail.com"
+    ];
+    
     # User-specific packages
     packages = with pkgs; [
       # Browsers & Communication