style: move public members first in class declarations

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

cbt/opengl/buffer.hpp

@@ -15,9 +15,6 @@ enum class buffer_type {
 };
 
 class buffer {
-    GLuint m_id = 0;
-    buffer_type m_type = buffer_type::vertex;
-
 public:
     buffer();
     explicit buffer(buffer_type type);
@@ -36,6 +33,10 @@ public:
     auto upload_dynamic(void const* data, size_t size) -> void;
     auto id() const -> GLuint;
     auto valid() const -> bool;
+
+private:
+    GLuint m_id = 0;
+    buffer_type m_type = buffer_type::vertex;
 };
 
 }