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.

CMakeLists.txt

@@ -20,6 +20,7 @@ find_package(glm REQUIRED)
 
 # OpenGL abstraction library
 add_library(cbt_opengl STATIC
+    "cbt/window.cpp"
     "cbt/opengl/context.cpp"
     "cbt/opengl/buffer.cpp"
     "cbt/opengl/texture.cpp"