build: modernize repo-lens skeleton and cross-platform configuration

Replace CPM with FetchContent and update GoogleTest to v1.18.0.
Add compiler toolchains and Debug, Release, and sanitizer presets.
Require CMake 4.0, use target properties for compiler settings,
and enable compilation database export.

Fix skeleton compilation errors and adopt trailing return types,
nodiscard execute methods, and noexcept signatures. Update CI
and project documentation without implementing any TODOs.

Validated Windows builds and tests in all three configurations.

test/CMakeLists.txt

@@ -1,14 +1,31 @@
 include(gtest)
 
 add_executable(
-  blank-slate-test
+  repo-lens-test
   test_main.cpp
 )
 
 target_link_libraries(
-  blank-slate-test
+  repo-lens-test
   gtest_main
 )
 
+if(ENABLE_ALL_REASONABLE_WARNINGS)
+  target_enable_warnings(repo-lens-test)
+endif()
+if(ENABLE_WARNINGS_AS_ERRORS)
+  target_warnings_as_errors(repo-lens-test)
+endif()
+if(ENABLE_SANITIZERS)
+  target_enable_sanitizers(repo-lens-test)
+  if(MSVC)
+    # @ai-bookmark MSVC_SANITIZER_ANNOTATION_ALIGNMENT
+    # MSVC STL sanitizer annotations must agree across linked translation units.
+    foreach(_test_dependency IN ITEMS gtest gtest_main gmock gmock_main)
+      target_enable_sanitizers(${_test_dependency})
+    endforeach()
+  endif()
+endif()
+
 include(GoogleTest)
-gtest_discover_tests(blank-slate-test)
+gtest_discover_tests(repo-lens-test)