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.

docker-compose.yml

@@ -0,0 +1,21 @@
+services:
+  minio:
+    image: minio/minio:latest
+    container_name: simples3-minio
+    ports:
+      - "9000:9000"   # API
+      - "9001:9001"   # Console
+    environment:
+      - MINIO_ROOT_USER=minioadmin
+      - MINIO_ROOT_PASSWORD=minioadmin
+    volumes:
+      - minio_data:/data
+    command: server /data --console-address ":9001"
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:9000/minio/health/live"]
+      interval: 30s
+      timeout: 20s
+      retries: 3
+
+volumes:
+  minio_data: