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/buffer.cpp

@@ -25,7 +25,7 @@ buffer::buffer(buffer&& other) noexcept
     other.m_id = 0;
 }
 
-buffer& buffer::operator=(buffer&& other) noexcept {
+auto buffer::operator=(buffer&& other) noexcept -> buffer& {
     if (this != &other) {
         if (m_id) {
             glDeleteBuffers(1, &m_id);

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);

cbt/opengl/texture.cpp

@@ -25,7 +25,7 @@ texture::texture(texture&& other) noexcept
     other.m_id = 0;
 }
 
-texture& texture::operator=(texture&& other) noexcept {
+auto texture::operator=(texture&& other) noexcept -> texture& {
     if (this != &other) {
         if (m_id) {
             glDeleteTextures(1, &m_id);

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);