fix(gateway): remap HERMES_HOME to target user in system service unit

When `sudo hermes gateway install --system --run-as-user <user>` generates
the systemd unit, get_hermes_home() resolves to /root/.hermes because
Path.home() returns root's home under sudo. The unit correctly sets
HOME= and User= via _system_service_identity(), but HERMES_HOME was
computed independently and pointed to root's config directory.

Add _hermes_home_for_target_user() which remaps the current HERMES_HOME
to the equivalent path under the target user's home. This handles:
- Default ~/.hermes → target user's ~/.hermes
- Profiles (e.g. ~/.hermes/profiles/coder) → preserves relative structure
- Custom paths (e.g. /opt/hermes) → kept as-is

Supersedes #3861 which only handled the default case and left profiles
broken (also flagged by Copilot review).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/hermes_cli/test_gateway_service.py

@@ -339,6 +339,102 @@ class TestDetectVenvDir:
         assert result is None
 
 
+class TestSystemUnitHermesHome:
+    """HERMES_HOME in system units must reference the target user, not root."""
+
+    def test_system_unit_uses_target_user_home_not_calling_user(self, monkeypatch):
+        # Simulate sudo: Path.home() returns /root, target user is alice
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.delenv("HERMES_HOME", raising=False)
+        monkeypatch.setattr(
+            gateway_cli, "_system_service_identity",
+            lambda run_as_user=None: ("alice", "alice", "/home/alice"),
+        )
+        monkeypatch.setattr(
+            gateway_cli, "_build_user_local_paths",
+            lambda home, existing: [],
+        )
+
+        unit = gateway_cli.generate_systemd_unit(system=True, run_as_user="alice")
+
+        assert 'HERMES_HOME=/home/alice/.hermes' in unit
+        assert '/root/.hermes' not in unit
+
+    def test_system_unit_remaps_profile_to_target_user(self, monkeypatch):
+        # Simulate sudo with a profile: HERMES_HOME was resolved under root
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.setenv("HERMES_HOME", "/root/.hermes/profiles/coder")
+        monkeypatch.setattr(
+            gateway_cli, "_system_service_identity",
+            lambda run_as_user=None: ("alice", "alice", "/home/alice"),
+        )
+        monkeypatch.setattr(
+            gateway_cli, "_build_user_local_paths",
+            lambda home, existing: [],
+        )
+
+        unit = gateway_cli.generate_systemd_unit(system=True, run_as_user="alice")
+
+        assert 'HERMES_HOME=/home/alice/.hermes/profiles/coder' in unit
+        assert '/root/' not in unit
+
+    def test_system_unit_preserves_custom_hermes_home(self, monkeypatch):
+        # Custom HERMES_HOME not under any user's home — keep as-is
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.setenv("HERMES_HOME", "/opt/hermes-shared")
+        monkeypatch.setattr(
+            gateway_cli, "_system_service_identity",
+            lambda run_as_user=None: ("alice", "alice", "/home/alice"),
+        )
+        monkeypatch.setattr(
+            gateway_cli, "_build_user_local_paths",
+            lambda home, existing: [],
+        )
+
+        unit = gateway_cli.generate_systemd_unit(system=True, run_as_user="alice")
+
+        assert 'HERMES_HOME=/opt/hermes-shared' in unit
+
+    def test_user_unit_unaffected_by_change(self):
+        # User-scope units should still use the calling user's HERMES_HOME
+        unit = gateway_cli.generate_systemd_unit(system=False)
+
+        hermes_home = str(gateway_cli.get_hermes_home().resolve())
+        assert f'HERMES_HOME={hermes_home}' in unit
+
+
+class TestHermesHomeForTargetUser:
+    """Unit tests for _hermes_home_for_target_user()."""
+
+    def test_remaps_default_home(self, monkeypatch):
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.delenv("HERMES_HOME", raising=False)
+
+        result = gateway_cli._hermes_home_for_target_user("/home/alice")
+        assert result == "/home/alice/.hermes"
+
+    def test_remaps_profile_path(self, monkeypatch):
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.setenv("HERMES_HOME", "/root/.hermes/profiles/coder")
+
+        result = gateway_cli._hermes_home_for_target_user("/home/alice")
+        assert result == "/home/alice/.hermes/profiles/coder"
+
+    def test_keeps_custom_path(self, monkeypatch):
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/root")))
+        monkeypatch.setenv("HERMES_HOME", "/opt/hermes")
+
+        result = gateway_cli._hermes_home_for_target_user("/home/alice")
+        assert result == "/opt/hermes"
+
+    def test_noop_when_same_user(self, monkeypatch):
+        monkeypatch.setattr(Path, "home", staticmethod(lambda: Path("/home/alice")))
+        monkeypatch.delenv("HERMES_HOME", raising=False)
+
+        result = gateway_cli._hermes_home_for_target_user("/home/alice")
+        assert result == "/home/alice/.hermes"
+
+
 class TestGeneratedUnitUsesDetectedVenv:
     def test_systemd_unit_uses_dot_venv_when_detected(self, tmp_path, monkeypatch):
         dot_venv = tmp_path / ".venv"