feat: implement Simple List API for S3 objects

- Add List method with ListInput and ListResponse structs following simples3 patterns
- Implement pagination, prefix filtering, and delimiter grouping
- Add comprehensive test suite for List functionality
- Update tests to use environment variables instead of hardcoded values
- Fix deprecated io/ioutil usage throughout codebase
- Add comprehensive README with API documentation and examples
- Add Nix development environment with flake.nix
- Add Justfile for modern command runner
- Add Docker Compose setup for local MinIO testing
- Update GitHub Actions to use Nix and Justfile for CI consistency
- Fix test naming and setup flow issues

New API:
s3.List(ListInput{
    Bucket: "my-bucket",
    Prefix: "documents/",
    MaxKeys: 100,
})

All tests pass with new simplified API design.

flake.nix

@@ -0,0 +1,62 @@
+{
+  description = "Go project for simples3 with ListObjectsV2 support";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forEachSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
+        pkgs = import nixpkgs {
+          inherit system;
+        };
+      });
+    in
+    {
+      devShells = forEachSystem ({ pkgs }: {
+        default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            # Go development
+            go
+            gopls
+            go-outline
+            delve
+
+            # Build tools and utilities
+            just
+            docker
+            awscli2
+
+            # Additional utilities
+            curl
+            jq
+          ];
+
+          # Environment variables for the shell
+          shellHook = ''
+            echo "🚀 SimpleS3 Development Environment Loaded"
+            echo "Go version: $(go version)"
+            echo "Available commands: just --list"
+            export GOPATH=$(go env GOPATH)
+            export PATH=$PATH:$GOPATH/bin
+
+            # Local S3 (MinIO) configuration for testing
+            export AWS_S3_BUCKET=testbucket
+            export AWS_S3_ENDPOINT=http://localhost:9000
+            export AWS_S3_REGION=us-east-1
+            export AWS_EC2_METADATA_DISABLED=true
+
+            # AWS CLI credentials for MinIO
+            export AWS_ACCESS_KEY_ID=minioadmin
+            export AWS_SECRET_ACCESS_KEY=minioadmin
+
+            # Simples3 library credentials
+            export AWS_S3_ACCESS_KEY=minioadmin
+            export AWS_S3_SECRET_KEY=minioadmin
+          '';
+        };
+      });
+    };
+}