feat: add OpenGL abstraction layer with RAII resources

- Replace window class with cbt::opengl::context
- Add buffer resource (VBO, EBO, UBO, SSBO) with move semantics
- Add texture resource with format/type enums and filtering
- Add descriptor_set for Vulkan-style resource binding
- All resources use RAII with proper cleanup

AGENTS.md

@@ -89,8 +89,15 @@ cuber/
   CMakeLists.txt      # Build configuration
   cuber.cpp           # Entry point
   cbt/                # Project namespace
-    window.hpp        # Window RAII wrapper
-    window.cpp        # Window implementation
+    opengl/           # OpenGL abstraction layer
+      context.hpp     # OpenGL context RAII wrapper
+      context.cpp     # Context implementation
+      buffer.hpp      # Buffer resource (VBO, EBO, UBO, SSBO)
+      buffer.cpp      # Buffer implementation
+      texture.hpp     # Texture resource
+      texture.cpp     # Texture implementation
+      descriptor.hpp  # Descriptor set (Vulkan-style binding)
+      descriptor.cpp  # Descriptor implementation
   deps/               # Custom Find*.cmake scripts
     Findfmt.cmake     # fmt library
 ```