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.

AGENTS.md

@@ -34,7 +34,7 @@ To add a new dependency:
 
 The project is split into static libraries:
 
-- **`cbt_opengl`** — OpenGL abstraction (context, buffer, texture, vao, shader, descriptor)
+- **`cbt_opengl`** — OpenGL abstraction and window management (window, context, buffer, texture, vao, shader, descriptor)
 - **`cbt_scene`** — Base scene class
 - **`scenes_cube`** — Cube scene implementation
 
@@ -102,8 +102,10 @@ cuber/
   cbt/                # Project namespace (utilities)
     scene.hpp         # Base scene class
     scene.cpp         # Scene implementation
+    window.hpp        # GLFW window RAII wrapper
+    window.cpp        # Window implementation
     opengl/           # OpenGL abstraction layer
-      context.hpp     # OpenGL context RAII wrapper
+      context.hpp     # OpenGL context RAII wrapper (GLAD setup)
       context.cpp     # Context implementation
       buffer.hpp      # Buffer resource (VBO, EBO, UBO, SSBO)
       buffer.cpp      # Buffer implementation