style: move public members first in class declarations

- Reorder all class headers to put public interface before private members
- Document convention in AGENTS.md

scenes/cube.hpp

@@ -11,6 +11,13 @@
 namespace cbt::scenes {
 
 class cube final : public scene {
+public:
+    cube();
+    auto init() -> bool override;
+    auto update(float delta_time) -> void override;
+    auto render() -> void override;
+
+private:
     opengl::shader m_prog;
     opengl::buffer m_vbo;
     opengl::vao m_vao;
@@ -23,12 +30,6 @@ class cube final : public scene {
 
     auto build_mesh() -> void;
     auto build_shader() -> bool;
-
-public:
-    cube();
-    auto init() -> bool override;
-    auto update(float delta_time) -> void override;
-    auto render() -> void override;
 };
 
 }