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.hpp

@@ -23,17 +23,17 @@ public:
     explicit buffer(buffer_type type);
     ~buffer();
 
-    buffer(const buffer&) = delete;
-    buffer& operator=(const buffer&) = delete;
+    buffer(buffer const&) = delete;
+    buffer& operator=(buffer const&) = delete;
 
     buffer(buffer&& other) noexcept;
     buffer& operator=(buffer&& other) noexcept;
 
     auto bind() const -> void;
     auto unbind() const -> void;
-    auto upload(const void* data, size_t size) -> void;
+    auto upload(void const* data, size_t size) -> void;
     auto upload(std::vector<uint8_t> data) -> void;
-    auto upload_dynamic(const void* data, size_t size) -> void;
+    auto upload_dynamic(void const* data, size_t size) -> void;
     auto id() const -> GLuint;
     auto valid() const -> bool;
 };