Double-start guard: launching marker, probe-and-adopt, fresh-dir relaunch; skip unreachable terminals per tick

Before the launch call the planned run (run id, terminal, handle, bookkeeping) is saved as a launching
marker. If the launch call fails or agentd dies before recording the run, the next tick probes the run
directory (exit.txt / launch.pid / pid.txt): evidence -> adopt the run; nothing -> relaunch with the same
run id in <run_id>-r2 (-r3 ...); probe unreachable -> keep the marker, which holds the terminal slot.
Transports refuse to clean a run directory holding a pid file. A terminal whose relay fails is skipped
for the rest of the tick (heartbeat.down) while other terminals carry on.

tests/test_posix_remote.py

@@ -203,3 +203,17 @@ class PosixRemoteTest(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+    def test_probe_dir_name_and_pid_guard(self):
+        ex = make_executor(self.cfg, "mac", "shell", self.t)
+        run = Run("GW-5-1", self.cfg.terminals["mac"]["repo"], "", script="sleep 0.2", dir_name="GW-5-1-r2")
+        self.assertEqual(ex.probe(ex.handle(run)), [])
+        h = ex.start(run)
+        self.assertEqual((os.path.basename(h["run_dir"]), h["task"]), ("GW-5-1-r2", "agentd-GW-5-1-r2"))
+        self.assertTrue(wait(ex, h))
+        self.assertEqual(ex.probe(h), ["exit.txt", "launch.pid", "pid.txt"])
+        n = len(self.t.calls)
+        with self.assertRaises(AssertionError):        # LocalBash: marker LAUNCHED= missing (HASPID)
+            ex.start(run)
+        self.assertIn("HASPID", self.t.calls[n])
+        self.assertTrue(os.path.exists(os.path.join(h["run_dir"], "exit.txt")))   # nothing wiped