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.cpp

@@ -0,0 +1,86 @@
+#ifdef _WIN32
+#include <dwmapi.h>
+#include <windows.h>
+#endif
+
+#define GLFW_INCLUDE_NONE
+#include "GLFW/glfw3.h"
+
+#ifdef _WIN32
+#define GLFW_EXPOSE_NATIVE_WIN32
+#include "GLFW/glfw3native.h"
+#endif
+
+#include "fmt/std.h"
+
+#include "cbt/window.hpp"
+
+namespace cbt {
+
+auto window::init_glfw() -> bool {
+    if (!glfwInit()) {
+        fmt::print("Failed to initialize GLFW\n");
+        return false;
+    }
+    return true;
+}
+
+auto window::terminate_glfw() -> void {
+    glfwTerminate();
+}
+
+window::window(std::string title, int width, int height) {
+    if (!init_glfw()) {
+        return;
+    }
+    m_glfw_initialized = true;
+
+    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
+    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 1);
+    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
+
+    m_window = glfwCreateWindow(width, height, title.c_str(), nullptr, nullptr);
+    if (!m_window) {
+        fmt::print("Failed to create window\n");
+        terminate_glfw();
+        m_glfw_initialized = false;
+        return;
+    }
+
+#ifdef _WIN32
+    auto hwnd = glfwGetWin32Window(m_window);
+    BOOL use_dark_mode = TRUE;
+    DwmSetWindowAttribute(hwnd, DWMWA_USE_IMMERSIVE_DARK_MODE, &use_dark_mode, sizeof(use_dark_mode));
+#endif
+}
+
+window::~window() {
+    if (m_window) {
+        glfwDestroyWindow(m_window);
+    }
+    if (m_glfw_initialized) {
+        terminate_glfw();
+    }
+}
+
+auto window::should_close() const -> bool {
+    return m_window && glfwWindowShouldClose(m_window);
+}
+
+auto window::valid() const -> bool {
+    return m_window != nullptr;
+}
+
+auto window::swap_buffers() -> void {
+    glfwSwapBuffers(m_window);
+}
+
+auto window::poll_events() -> void {
+    glfwPollEvents();
+}
+
+auto window::raw() const -> GLFWwindow* {
+    return m_window;
+}
+
+}