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.

cuber.cpp

@@ -5,11 +5,18 @@
 #include "GLFW/glfw3.h"
 #include "asio.hpp"
 
+#include "cbt/window.hpp"
 #include "cbt/opengl/context.hpp"
 #include "scenes/cube.hpp"
 
 auto main(int, char const*[]) -> int {
-    auto ctx = cbt::opengl::context("cuber", 1280, 720);
+    auto win = cbt::window("cuber", 1280, 720);
+
+    if (!win.valid()) {
+        return 1;
+    }
+
+    auto ctx = cbt::opengl::context(win);
 
     if (!ctx.valid()) {
         return 1;
@@ -37,10 +44,10 @@ auto main(int, char const*[]) -> int {
     // render loop
     auto prev = std::chrono::steady_clock::now();
 
-    while (!ctx.should_close()) {
+    while (!win.should_close()) {
         process_signals();
 
-        if (quit || glfwGetKey(ctx.raw(), GLFW_KEY_Q) == GLFW_PRESS) {
+        if (quit || glfwGetKey(win.raw(), GLFW_KEY_Q) == GLFW_PRESS) {
             break;
         }
 
@@ -51,8 +58,8 @@ auto main(int, char const*[]) -> int {
         scn.update(dt);
         scn.render();
 
-        ctx.swap_buffers();
-        ctx.poll_events();
+        win.swap_buffers();
+        win.poll_events();
     }
 
     return 0;