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/shader.cpp

@@ -20,7 +20,7 @@ shader::shader(shader&& other) noexcept
     other.m_id = 0;
 }
 
-shader& shader::operator=(shader&& other) noexcept {
+auto shader::operator=(shader&& other) noexcept -> shader& {
     if (this != &other) {
         if (m_id) {
             glDeleteProgram(m_id);