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/buffer.cpp

@@ -45,7 +45,7 @@ auto buffer::unbind() const -> void {
     glBindBuffer(static_cast<GLenum>(m_type), 0);
 }
 
-auto buffer::upload(const void* data, size_t size) -> void {
+auto buffer::upload(void const* data, size_t size) -> void {
     bind();
     glBufferData(static_cast<GLenum>(m_type), size, data, GL_STATIC_DRAW);
     unbind();
@@ -55,7 +55,7 @@ auto buffer::upload(std::vector<uint8_t> data) -> void {
     upload(data.data(), data.size());
 }
 
-auto buffer::upload_dynamic(const void* data, size_t size) -> void {
+auto buffer::upload_dynamic(void const* data, size_t size) -> void {
     bind();
     glBufferData(static_cast<GLenum>(m_type), size, data, GL_DYNAMIC_DRAW);
     unbind();