feat: add ProcessExecutor, FAKE mode, fix thread lifecycle crash

- Add ProcessExecutor class wrapping PIProcess with proper cleanup
- Add CMake FAKE option (-DFAKE=ON) for testing without opencode
- Add 3 FAKE-mode tests for full pipeline execution, status transitions, output
- Fix PIThread lifecycle: use startOnce() instead of start(), track threads,
  wait + delete in destructor to prevent use-after-free of PIDeque<PIChar>
- Update build.sh with --fake flag and automatic cache invalidation
- Upgrade CMake to C++17 standard
- Update AGENTS.md with build commands and FAKE mode

pipeline-runner/CMakeLists.txt

@@ -1,10 +1,18 @@
 cmake_minimum_required(VERSION 3.10)
 
 project(PipelineRunner)
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
+option(FAKE "Fake mode for testing without opencode" OFF)
+
+if(FAKE)
+  message(STATUS "FAKE mode ON — using fake process executor")
+else()
+  message(STATUS "FAKE mode OFF — using real process executor")
+endif()
+
 # PIP
 find_package(PIP REQUIRED)
 
@@ -28,6 +36,9 @@ set(SRCS ${ALL_SRCS})
 add_executable(pipeline-runner src/main.cpp ${SRCS} ${HDRS})
 target_include_directories(pipeline-runner PRIVATE src)
 target_link_libraries(pipeline-runner PIP PIP::HTTPServer PIP::Crypt PIP::Console)
+if(FAKE)
+	target_compile_definitions(pipeline-runner PRIVATE FAKE)
+endif()
 
 set_target_properties(pipeline-runner PROPERTIES
     INSTALL_RPATH "\$ORIGIN;\$ORIGIN/lib"
@@ -38,6 +49,9 @@ set_target_properties(pipeline-runner PROPERTIES
 add_executable(test-pipeline-runner tests/test_pipeline.cpp tests/test_runner.cpp ${SRCS})
 target_include_directories(test-pipeline-runner PRIVATE src tests)
 target_link_libraries(test-pipeline-runner PIP PIP::HTTPServer PIP::Crypt PIP::Console GTest::gtest_main)
+if(FAKE)
+	target_compile_definitions(test-pipeline-runner PRIVATE FAKE)
+endif()
 
 include(GoogleTest)
 gtest_discover_tests(test-pipeline-runner)