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/shader.hpp

@@ -13,14 +13,14 @@ public:
     shader();
     ~shader();
 
-    shader(const shader&) = delete;
-    shader& operator=(const shader&) = delete;
+    shader(shader const&) = delete;
+    shader& operator=(shader const&) = delete;
 
     shader(shader&& other) noexcept;
     shader& operator=(shader&& other) noexcept;
 
-    auto compile_vertex(const char* source) -> bool;
-    auto compile_fragment(const char* source) -> bool;
+    auto compile_vertex(char const* source) -> bool;
+    auto compile_fragment(char const* source) -> bool;
     auto link() -> bool;
     auto use() const -> void;
     auto unuse() const -> void;