fix(gateway): close --replace race completely by claiming PID before adapter startup

Follow-up on top of opriz's atomic PID file fix. The prior change caught
the race AFTER runner.start(), so the loser still opened Telegram polling
and Discord gateway sockets before detecting the conflict and exiting.

Hoist the PID-claim block to BEFORE runner.start(). Now the loser of the
O_CREAT|O_EXCL race returns from start_gateway() without ever bringing up
any platform adapter — no Telegram conflict, no Discord duplicate session.

Also add regression tests:
- test_write_pid_file_is_atomic_against_concurrent_writers: second
  write_pid_file() raises FileExistsError rather than clobbering.
- Two existing replace-path tests updated to stateful mocks since the
  real post-kill state (get_running_pid None after remove_pid_file)
  is now exercised by the hoisted re-check.

tests/gateway/test_status.py

@@ -19,6 +19,30 @@ class TestGatewayPidState:
         assert isinstance(payload["argv"], list)
         assert payload["argv"]
 
+    def test_write_pid_file_is_atomic_against_concurrent_writers(self, tmp_path, monkeypatch):
+        """Regression: two concurrent --replace invocations must not both win.
+
+        Without O_CREAT|O_EXCL, two processes racing through start_gateway()'s
+        termination-wait would both write to gateway.pid, silently overwriting
+        each other and leaving multiple gateway instances alive (#11718).
+        """
+        import pytest
+
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+        # First write wins.
+        status.write_pid_file()
+        assert (tmp_path / "gateway.pid").exists()
+
+        # Second write (simulating a racing --replace that missed the earlier
+        # guards) must raise FileExistsError rather than clobber the record.
+        with pytest.raises(FileExistsError):
+            status.write_pid_file()
+
+        # Original record is preserved.
+        payload = json.loads((tmp_path / "gateway.pid").read_text())
+        assert payload["pid"] == os.getpid()
+
     def test_get_running_pid_rejects_live_non_gateway_pid(self, tmp_path, monkeypatch):
         monkeypatch.setenv("HERMES_HOME", str(tmp_path))
         pid_path = tmp_path / "gateway.pid"