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/texture.cpp

@@ -46,7 +46,7 @@ auto texture::unbind() -> void {
     glBindTexture(static_cast<GLenum>(m_target), 0);
 }
 
-auto texture::upload(const void* data, int width, int height, texture_format format, texture_type type) -> void {
+auto texture::upload(void const* data, int width, int height, texture_format format, texture_type type) -> void {
     bind();
     glTexImage2D(static_cast<GLenum>(m_target), 0,
                  static_cast<GLenum>(format), width, height, 0,