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/nixos/services/base/default.nix

@@ -62,6 +62,7 @@ in
           rrsync
           s-tui
           sbctl
+          ssh-tresor
           sops
           strace
           tmux