Address review: remove CMake test scripts, don't SYSTEM-include lefticus tools

Remove check_system_includes.cmake.in and associated test from
test/CMakeLists.txt — these would confuse end users. Remove SYSTEM YES
from the lefticus/tools CPMAddPackage call so its headers are not
treated as system includes.

Requested by @lefticus in PR #145 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: leftibot

Dependencies.cmake

@@ -77,9 +77,7 @@ function(myproject_setup_dependencies)
       GITHUB_REPOSITORY
       "lefticus/tools"
       GIT_TAG
-      "main"
-      SYSTEM
-      YES)
+      "main")
   endif()
 
 endfunction()

test/CMakeLists.txt

@@ -21,20 +21,6 @@ endif()
 
 include(${Catch2_SOURCE_DIR}/extras/Catch.cmake)
 
-# Regression test for issue #143: verify CPM dependencies use SYSTEM includes
-get_property(_fmt_dir TARGET fmt::fmt PROPERTY SOURCE_DIR)
-if(_fmt_dir)
-  get_property(_fmt_dir_system DIRECTORY "${_fmt_dir}" PROPERTY SYSTEM)
-else()
-  set(_fmt_dir_system FALSE)
-endif()
-configure_file(
-  "${CMAKE_CURRENT_SOURCE_DIR}/check_system_includes.cmake.in"
-  "${CMAKE_CURRENT_BINARY_DIR}/check_system_includes.cmake"
-  @ONLY)
-add_test(NAME cmake.system_includes
-         COMMAND ${CMAKE_COMMAND} -P "${CMAKE_CURRENT_BINARY_DIR}/check_system_includes.cmake")
-
 # Provide a simple smoke test to make sure that the CLI works and can display a --help message
 add_test(NAME cli.has_help COMMAND intro --help)