Let ament handle export installation in ROS2 builds

When using ament, ament_package() should handle installing the export
files and generating the Config.cmake. The explicit install(EXPORT ...)
was interfering with this, causing find_package(reactor-cpp) to fail
in downstream packages.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lsk567

lib/CMakeLists.txt

@@ -66,5 +66,9 @@ if(REACTOR_CPP_INSTALL)
       LIBRARY  DESTINATION "${CMAKE_INSTALL_LIBDIR}" OPTIONAL
       RUNTIME  DESTINATION "${CMAKE_INSTALL_BINDIR}" OPTIONAL)
 
-  install(EXPORT ${LIB_TARGET}Targets DESTINATION share/${LIB_TARGET}/cmake)
+  # Only install export file manually when not using ament.
+  # When using ament, ament_package() handles export installation.
+  if(NOT REACTOR_CPP_USE_AMENT)
+    install(EXPORT ${LIB_TARGET}Targets DESTINATION share/${LIB_TARGET}/cmake)
+  endif()
 endif()