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.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1757745802,
+        "narHash": "sha256-hLEO2TPj55KcUFUU1vgtHE9UEIOjRcH/4QbmfHNF820=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c23193b943c6c689d70ee98ce3128239ed9e32d1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}