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/src/processexecutor.h

@@ -0,0 +1,20 @@
+#ifndef PROCESSEXECUTOR_H
+#define PROCESSEXECUTOR_H
+
+#include <pistring.h>
+#include <pistringlist.h>
+
+struct ProcessResult {
+	int exitCode;
+	PIString output;
+	PIString error;
+};
+
+class ProcessExecutor {
+public:
+	// Execute a command with args, wait up to timeoutSeconds, return result.
+	// Only one process runs at a time — synchronous, stack-based PIProcess.
+	static ProcessResult run(const PIString & command, const PIStringList & args, int timeoutSeconds, const PIString & workingDir);
+};
+
+#endif