refactor: separate window from opengl::context

Extract GLFW window management into a dedicated cbt::window
class (new files in cbt/). The opengl::context now only
handles GLAD setup and context activation (no more window
creation or GLFW init/terminate).

Updated main loop in cuber.cpp, CMakeLists.txt (to build
the new source), and AGENTS.md (docs + source layout).

Addresses the design note in context.cpp about mixing
concerns.

cbt/opengl/context.hpp

@@ -1,29 +1,19 @@
 #pragma once
 
-#include <string>
-
-#define GLFW_INCLUDE_NONE
-#include "GLFW/glfw3.h"
+#include "cbt/window.hpp"
 
 namespace cbt::opengl {
 
 class context {
 public:
-    explicit context(std::string title, int width, int height);
+    explicit context(window const& win);
     ~context();
 
-    auto should_close() const -> bool;
     auto valid() const -> bool;
-    auto swap_buffers() -> void;
-    auto poll_events() -> void;
-    auto raw() const -> GLFWwindow*;
 
 private:
-    GLFWwindow* m_window = nullptr;
-    bool m_initialized = false;
+    bool m_valid = false;
 
-    static auto init() -> bool;
-    static auto terminate() -> void;
     static auto setup_gl() -> bool;
     static auto print_info() -> void;
 };