feat(gateway): multiplex phase 4 — lifecycle guard + per-profile observability

- _guard_named_profile_under_multiplexer: when the default gateway is running
  with gateway.multiplex_profiles=on, a named-profile 'hermes gateway run' hard
  -errors (pointing at the multiplexer) instead of double-binding that
  profile's platforms. Inert unless all hold: this invocation is a named
  profile, a default-profile gateway is alive, and its config has multiplexing
  on. --force overrides. Wired into run_gateway's guard chain.
- write_runtime_status gains served_profiles: the secondary-adapter startup
  records [active] + multiplexed profiles into runtime_status.json so
  'hermes status' can show per-profile coverage without a second probe. Absent
  for single-profile gateways.

Tests: served_profiles round-trips and is absent by default; guard is inert for
the default profile / under --force / when no default gateway is running.

tests/gateway/test_multiplex_lifecycle.py

@@ -0,0 +1,55 @@
+"""Phase 4: lifecycle guard + per-profile observability."""
+import pytest
+
+
+class TestServedProfilesStatus:
+    def test_write_and_read_served_profiles(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        import importlib
+        import gateway.status as status
+        importlib.reload(status)
+        try:
+            status.write_runtime_status(
+                gateway_state="running", served_profiles=["default", "coder"]
+            )
+            rec = status.read_runtime_status()
+            assert rec.get("served_profiles") == ["default", "coder"]
+        finally:
+            importlib.reload(status)
+
+    def test_served_profiles_absent_by_default(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        import importlib
+        import gateway.status as status
+        importlib.reload(status)
+        try:
+            status.write_runtime_status(gateway_state="running")
+            rec = status.read_runtime_status()
+            assert "served_profiles" not in rec
+        finally:
+            importlib.reload(status)
+
+
+class TestNamedProfileMultiplexerGuard:
+    """_guard_named_profile_under_multiplexer is inert unless all conditions hold."""
+
+    def test_inert_for_default_profile(self, monkeypatch):
+        from hermes_cli import gateway as gw
+        monkeypatch.setattr(gw, "_profile_suffix", lambda: "")
+        # Should return without raising (default profile => guard N/A).
+        gw._guard_named_profile_under_multiplexer(force=False)
+
+    def test_force_bypasses(self, monkeypatch):
+        from hermes_cli import gateway as gw
+        # Even if it looks like a named profile, force returns immediately.
+        monkeypatch.setattr(gw, "_profile_suffix", lambda: "coder")
+        gw._guard_named_profile_under_multiplexer(force=True)
+
+    def test_inert_when_no_default_gateway_running(self, monkeypatch, tmp_path):
+        from hermes_cli import gateway as gw
+        monkeypatch.setattr(gw, "_profile_suffix", lambda: "coder")
+        monkeypatch.setattr(
+            "hermes_constants.get_default_hermes_root", lambda: tmp_path
+        )
+        # No gateway.pid in tmp_path => no running default gateway => no raise.
+        gw._guard_named_profile_under_multiplexer(force=False)