fix(gateway): anchor service WorkingDirectory at HERMES_HOME, not the source checkout

The systemd unit (and launchd plist) pinned WorkingDirectory to PROJECT_ROOT
(the checkout the unit was generated from). When that checkout is transient —
a git worktree, or a clone hermes update later relocates/removes — the path
rots. systemd then fails the start at the CHDIR step (status=200/CHDIR) BEFORE
Python loads, so the on-boot refresh_systemd_unit_if_needed() self-heal never
runs and Restart=always crash-loops forever on a dead directory. Observed in
the wild: a gateway that crash-looped 153 times overnight, bot offline until a
manual 'hermes gateway restart' regenerated the unit.

Anchor cwd at HERMES_HOME instead — it never moves, always exists, and the
gateway never needed cwd to be the checkout (ExecStart uses an absolute python
+ -m hermes_cli.main). Existing broken units now differ from the generated unit
and self-heal on the next start/restart/update.

tests/hermes_cli/test_gateway_service.py

@@ -2531,3 +2531,46 @@ class TestGatewayCommandCatchesSystemScopeError:
         # Renders the message, NOT the ``('msg', 'action')`` tuple repr
         assert "System gateway start requires root. Re-run with sudo." in out
         assert "('" not in out  # no tuple repr leaking through
+
+
+class TestServiceWorkingDirIsStable:
+    """The gateway service must anchor WorkingDirectory at a stable path
+    (HERMES_HOME), never the source checkout / worktree, so a relocated or
+    deleted checkout can't crash-loop the unit on CHDIR (status=200).
+    """
+
+    def test_stable_working_dir_uses_hermes_home(self, tmp_path, monkeypatch):
+        home = tmp_path / ".hermes"
+        home.mkdir()
+        monkeypatch.setattr(gateway_cli, "get_hermes_home", lambda: home)
+        assert Path(gateway_cli._stable_service_working_dir()) == home.resolve()
+
+    def test_stable_working_dir_falls_back_to_project_root(self, tmp_path, monkeypatch):
+        # HERMES_HOME points somewhere that does not exist -> fall back.
+        missing = tmp_path / "does-not-exist" / ".hermes"
+        monkeypatch.setattr(gateway_cli, "get_hermes_home", lambda: missing)
+        assert gateway_cli._stable_service_working_dir() == str(gateway_cli.PROJECT_ROOT)
+
+    def test_user_unit_workingdirectory_is_hermes_home_not_checkout(self, tmp_path, monkeypatch):
+        home = tmp_path / ".hermes"
+        home.mkdir()
+        monkeypatch.setattr(gateway_cli, "get_hermes_home", lambda: home)
+        unit = gateway_cli.generate_systemd_unit(system=False)
+        wd = [l for l in unit.splitlines() if l.startswith("WorkingDirectory=")]
+        assert wd, "unit has no WorkingDirectory line"
+        value = wd[0].split("=", 1)[1]
+        assert Path(value).resolve() == home.resolve()
+        # The bug class: never pin cwd inside a transient worktree checkout.
+        assert "/.worktrees/" not in value
+
+    def test_launchd_workingdirectory_is_hermes_home(self, tmp_path, monkeypatch):
+        import re
+
+        home = tmp_path / ".hermes"
+        home.mkdir()
+        monkeypatch.setattr(gateway_cli, "get_hermes_home", lambda: home)
+        plist = gateway_cli.generate_launchd_plist()
+        m = re.search(r"<key>WorkingDirectory</key>\s*<string>(.*?)</string>", plist)
+        assert m, "plist has no WorkingDirectory entry"
+        assert Path(m.group(1)).resolve() == home.resolve()
+        assert "/.worktrees/" not in m.group(1)