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.cpp

@@ -0,0 +1,30 @@
+#include "processexecutor.h"
+
+#include <pibytearray.h>
+#include <piprocess.h>
+#include <pisystemtime.h>
+
+ProcessResult ProcessExecutor::run(const PIString & command, const PIStringList & args, int timeoutSeconds, const PIString & workingDir) {
+	PIProcess proc;
+	proc.enableReadStdOut(true);
+	proc.enableReadStdErr(true);
+
+	if (!workingDir.isEmpty()) {
+		proc.setWorkingDirectory(workingDir);
+	}
+
+	proc.exec(command, args);
+	proc.waitForFinish(PISystemTime::fromSeconds(timeoutSeconds));
+
+	PIByteArray stdoutData = proc.readOutput();
+	PIByteArray stderrData = proc.readError();
+	int rc                 = proc.exitCode();
+
+	proc.stopAndWait();
+
+	ProcessResult result;
+	result.exitCode = rc;
+	result.output   = PIString::fromUTF8(stdoutData);
+	result.error    = PIString::fromUTF8(stderrData);
+	return result;
+}