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.
portersky
22d2bb1c40