fix(gateway): refuse to write service definitions with a temp-dir HERMES_HOME (#44267)

* fix(gateway): refuse to write service definitions with a temp-dir HERMES_HOME

A test/E2E harness that exports HERMES_HOME=/tmp/... and touches any
gateway service write path (install, start self-heal, restart's
refresh_systemd_unit_if_needed) bakes the throwaway home into the
production systemd unit / launchd plist. The gateway then restarts
'healthy' but pointed at an empty temp home — no platforms enabled,
deaf to every message (live incident 2026-06-11: /tmp/hermes-e2e-41264
poisoned the unit during a PR-review E2E probe; the post-update restart
produced a 7-hour zombie gateway).

The existing safety belt only sniffed pytest-shaped markers
(/pytest-of-, /hermes_test). Add a structural guard:
_temp_home_in_service_definition() extracts HERMES_HOME from the
generated systemd unit or launchd plist and refuses the write (with
actionable guidance) when it resolves under tempfile.gettempdir(),
/tmp, /var/tmp, or the macOS /private variants. Wired into all five
write sites: systemd refresh + install, launchd refresh + install +
start self-heal.

* test: patch unit generator in install tests tripped by temp-home guard

CI runs hermetic with HERMES_HOME under a tmp dir, so the real
generate_systemd_unit() output now (correctly) trips the new temp-home
write guard in three install tests. Patch the generator with synthetic
non-temp content — same pattern the existing pytest-marker guard tests
use.

tests/hermes_cli/test_gateway.py

@@ -369,6 +369,16 @@ def test_systemd_install_checks_linger_status(monkeypatch, tmp_path, capsys):
     unit_path = tmp_path / "systemd" / "user" / "hermes-gateway.service"
 
     monkeypatch.setattr(gateway, "get_systemd_unit_path", lambda system=False: unit_path)
+    # Synthetic unit with a non-temp home: the real generator bakes the
+    # hermetic test HERMES_HOME (a tmp dir), which the temp-home write
+    # guard correctly refuses.
+    monkeypatch.setattr(
+        gateway,
+        "generate_systemd_unit",
+        lambda system=False, run_as_user=None: (
+            '[Service]\nEnvironment="HERMES_HOME=/home/alice/.hermes"\n'
+        ),
+    )
 
     calls = []
     helper_calls = []
@@ -396,6 +406,15 @@ def test_systemd_install_can_skip_enable_on_startup(monkeypatch, tmp_path, capsy
     unit_path = tmp_path / "systemd" / "user" / "hermes-gateway.service"
 
     monkeypatch.setattr(gateway, "get_systemd_unit_path", lambda system=False: unit_path)
+    # Non-temp home so the temp-home write guard (which trips on the
+    # hermetic test HERMES_HOME) stays out of the way.
+    monkeypatch.setattr(
+        gateway,
+        "generate_systemd_unit",
+        lambda system=False, run_as_user=None: (
+            '[Service]\nEnvironment="HERMES_HOME=/home/alice/.hermes"\n'
+        ),
+    )
 
     calls = []
     helper_calls = []