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/shader.hpp

@@ -7,8 +7,6 @@
 namespace cbt::opengl {
 
 class shader {
-    GLuint m_id = 0;
-
 public:
     shader();
     ~shader();
@@ -26,6 +24,9 @@ public:
     auto unuse() const -> void;
     auto id() const -> GLuint;
     auto valid() const -> bool;
+
+private:
+    GLuint m_id = 0;
 };
 
 }