feat: render spinning 3D cube with glm

- Add glm dependency for matrix transformations
- Replace triangle with 6-face colored cube
- Add MVP shader uniforms (model, view, projection)
- Enable depth testing for proper 3D rendering
- Spin cube around (1, 0.5, 0.3) axis

style: fix trailing return type for move assignment operators

- buffer/texture/shader/vao: use auto fn() -> T& style
- Document trailing return type convention in AGENTS.md

cbt/opengl/vao.cpp

@@ -17,7 +17,7 @@ vao::vao(vao&& other) noexcept
     other.m_id = 0;
 }
 
-vao& vao::operator=(vao&& other) noexcept {
+auto vao::operator=(vao&& other) noexcept -> vao& {
     if (this != &other) {
         if (m_id) {
             glDeleteVertexArrays(1, &m_id);