shells: convert to self-contained flake-parts module

Move devShells.nix to shells/default.nix as a flake-parts module
and use import-tree for automatic module discovery.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

shells/default.nix

@@ -0,0 +1,16 @@
+{ inputs, ... }:
+
+{
+  perSystem =
+    { pkgs, system, ... }:
+    {
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          inputs.agenix.packages.${system}.default
+          deploy-rs
+          nil
+          nixfmt
+        ];
+      };
+    };
+}