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.cpp

@@ -31,7 +31,7 @@ shader& shader::operator=(shader&& other) noexcept {
     return *this;
 }
 
-auto shader::compile_vertex(const char* source) -> bool {
+auto shader::compile_vertex(char const* source) -> bool {
     auto vert = glCreateShader(GL_VERTEX_SHADER);
     glShaderSource(vert, 1, &source, nullptr);
     glCompileShader(vert);
@@ -55,7 +55,7 @@ auto shader::compile_vertex(const char* source) -> bool {
     return true;
 }
 
-auto shader::compile_fragment(const char* source) -> bool {
+auto shader::compile_fragment(char const* source) -> bool {
     auto frag = glCreateShader(GL_FRAGMENT_SHADER);
     glShaderSource(frag, 1, &source, nullptr);
     glCompileShader(frag);