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

cuber.cpp

@@ -19,7 +19,7 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
     // compile shader
     auto prog = cbt::opengl::shader();
 
-    const char* vert_src = R"glsl(
+    char const* vert_src = R"glsl(
         #version 410 core
         layout(location = 0) in vec3 a_pos;
         layout(location = 1) in vec3 a_color;
@@ -30,7 +30,7 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
         }
     )glsl";
 
-    const char* frag_src = R"glsl(
+    char const* frag_src = R"glsl(
         #version 410 core
         in vec3 v_color;
         out vec4 frag_color;