feat(nix): add ssh-tresor package and flake input

- Included `ssh-tresor` in the package lists for Darwin and NixOS modules.
- Added `ssh-tresor` as a new flake input in `flake.nix` with its overlay.
- Integrates secure SSH key management functionality across configurations.

modules/darwin/services/base/default.nix

@@ -37,6 +37,7 @@ in
         tmux
         vim
         wget
+        ssh-tresor
         starship
       ];
       shells = [