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/window.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <string>
+
+#define GLFW_INCLUDE_NONE
+#include "GLFW/glfw3.h"
+
+namespace cbt {
+
+class window {
+public:
+    explicit window(std::string title, int width, int height);
+    ~window();
+
+    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_glfw_initialized = false;
+
+    static auto init_glfw() -> bool;
+    static auto terminate_glfw() -> void;
+};
+
+}