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

AGENTS.md

@@ -42,6 +42,7 @@ to the custom scripts instead of system-installed packages.
 - **4-space indentation**
 - **No semicolons after closing braces** for namespaces/classes
 - `auto` for obvious types (e.g. `auto main(...) -> int`)
+- **East const** (e.g. `char const*` not `const char*`)
 - `<>` includes only for system headers (std, OS, etc.)
 - `""` includes for third-party dependencies (e.g. `fmt`, `nlohmann/json`)
 - **Naming:** `snake_case` for variables, functions, and classes