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.

flake.nix

@@ -69,6 +69,9 @@
     nixos-hardware.url = "github:NixOS/nixos-hardware/master";
 
     xremap-flake.url = "github:xremap/nix-flake";
+
+    ssh-tresor.url = "github:haraldh/ssh-tresor";
+    ssh-tresor.inputs.nixpkgs.follows = "nixpkgs";
   };
 
   outputs =
@@ -113,6 +116,7 @@
         (final: prev: {
           inherit (cratedocs.packages.${prev.stdenv.hostPlatform.system}) cratedocs-mcp;
         })
+        ssh-tresor.overlays.default
       ];
 
       outputs-builder = channels: {