style: switch to east const across the codebase

- const T* -> T const* in all headers and implementations
- const T& -> T const& for copy constructor/operator= deletes
- update AGENTS.md to document east const convention

cbt/opengl/vao.hpp

@@ -11,8 +11,8 @@ public:
     vao();
     ~vao();
 
-    vao(const vao&) = delete;
-    vao& operator=(const vao&) = delete;
+    vao(vao const&) = delete;
+    vao& operator=(vao const&) = delete;
 
     vao(vao&& other) noexcept;
     vao& operator=(vao&& other) noexcept;