feat: add coverage and command doc rule

Add Coverage.cmake with gcovr integration, detecting Clang vs
GCC and wrapping llvm-cov gcov in a build-dir script to handle
paths with spaces on Windows. Update README and AGENTS.md with
the new one-command-per-block documentation rule.

CMakeLists.txt

@@ -9,6 +9,8 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/deps")
 include(Platform)
 include(Flags)
 
+include(Coverage)
+
 # Example Library
 add_subdirectory(ctdd)