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/build.sh

@@ -0,0 +1,58 @@
+#!/bin/bash
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+BUILD_DIR="${SCRIPT_DIR}/build"
+
+REBUILD=false
+FAKE=false
+for arg in "$@"; do
+  case "$arg" in
+    --rebuild) REBUILD=true ;;
+    --fake)    FAKE=true ;;
+  esac
+done
+
+CMAKE_ARGS=""
+if [ "${FAKE}" = true ]; then
+  CMAKE_ARGS="-DFAKE=ON"
+fi
+
+if [ "${REBUILD}" = true ]; then
+  echo "==> Clean build directory"
+  rm -rf "${BUILD_DIR}"
+  mkdir -p "${BUILD_DIR}"
+  echo "==> Configure"
+  cmake -S "${SCRIPT_DIR}" -B "${BUILD_DIR}" ${CMAKE_ARGS}
+fi
+
+mkdir -p "${BUILD_DIR}"
+
+if [ -f "${BUILD_DIR}/CMakeCache.txt" ]; then
+  CACHED_FAKE=$(grep -o "FAKE:BOOL=.\+" "${BUILD_DIR}/CMakeCache.txt" 2>/dev/null || echo "")
+  if [ "${FAKE}" = true ] && [ "${CACHED_FAKE}" != "FAKE:BOOL=ON" ]; then
+    echo "==> FAKE flag changed, cleaning build"
+    rm -rf "${BUILD_DIR}"
+    mkdir -p "${BUILD_DIR}"
+  elif [ "${FAKE}" = false ] && [ "${CACHED_FAKE}" = "FAKE:BOOL=ON" ]; then
+    echo "==> FAKE flag changed, cleaning build"
+    rm -rf "${BUILD_DIR}"
+    mkdir -p "${BUILD_DIR}"
+  fi
+fi
+
+if [ ! -f "${BUILD_DIR}/CMakeCache.txt" ]; then
+  echo "==> Configure"
+  cmake -S "${SCRIPT_DIR}" -B "${BUILD_DIR}" ${CMAKE_ARGS}
+fi
+
+echo "==> Build"
+cmake --build "${BUILD_DIR}" -j$(nproc)
+
+echo "==> Run tests"
+rm -rf /tmp/pipeline-runner-test/
+"${BUILD_DIR}/test-pipeline-runner" \
+  --gtest_brief=1 \
+  --gtest_output=xml:"${BUILD_DIR}/test-results.xml"
+
+echo "==> Done"