fix(skills): factor HERMES_HOME resolution into shared _hermes_home helper

The three google-workspace scripts (setup.py, google_api.py, gws_bridge.py)
each had their own way of resolving HERMES_HOME:

- setup.py imported hermes_constants (crashes outside Hermes process)
- google_api.py used os.getenv inline (no strip, no empty handling)
- gws_bridge.py defined its own local get_hermes_home() (duplicate)

Extract the common logic into _hermes_home.py which:
- Delegates to hermes_constants when available (profile support, etc.)
- Falls back to os.getenv with .strip() + empty-as-unset handling
- Provides display_hermes_home() with ~/ shortening for profiles

All three scripts now import from _hermes_home instead of duplicating.

7 regression tests cover the fallback path: env var override, default
~/.hermes, empty env var, display shortening, profile paths, and
custom non-home paths.

Closes #12722

tests/skills/test_google_oauth_setup.py

@@ -240,3 +240,69 @@ class TestExchangeAuthCode:
         assert setup_module.TOKEN_PATH.exists()
         # Pending auth is cleaned up
         assert not setup_module.PENDING_AUTH_PATH.exists()
+
+
+class TestHermesConstantsFallback:
+    """Tests for _hermes_home.py fallback when hermes_constants is unavailable."""
+
+    HELPER_PATH = (
+        Path(__file__).resolve().parents[2]
+        / "skills/productivity/google-workspace/scripts/_hermes_home.py"
+    )
+
+    def _load_helper(self, monkeypatch):
+        """Load _hermes_home.py with hermes_constants blocked."""
+        monkeypatch.setitem(sys.modules, "hermes_constants", None)
+        spec = importlib.util.spec_from_file_location("_hermes_home_test", self.HELPER_PATH)
+        module = importlib.util.module_from_spec(spec)
+        assert spec.loader is not None
+        spec.loader.exec_module(module)
+        return module
+
+    def test_fallback_uses_hermes_home_env_var(self, monkeypatch, tmp_path):
+        """When hermes_constants is missing, HERMES_HOME comes from env var."""
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "custom-hermes"))
+        module = self._load_helper(monkeypatch)
+        assert module.get_hermes_home() == tmp_path / "custom-hermes"
+
+    def test_fallback_defaults_to_dot_hermes(self, monkeypatch):
+        """When hermes_constants is missing and HERMES_HOME unset, default to ~/.hermes."""
+        monkeypatch.delenv("HERMES_HOME", raising=False)
+        module = self._load_helper(monkeypatch)
+        assert module.get_hermes_home() == Path.home() / ".hermes"
+
+    def test_fallback_ignores_empty_hermes_home(self, monkeypatch):
+        """Empty/whitespace HERMES_HOME is treated as unset."""
+        monkeypatch.setenv("HERMES_HOME", "  ")
+        module = self._load_helper(monkeypatch)
+        assert module.get_hermes_home() == Path.home() / ".hermes"
+
+    def test_fallback_display_hermes_home_shortens_path(self, monkeypatch):
+        """Fallback display_hermes_home() uses ~/ shorthand like the real one."""
+        monkeypatch.delenv("HERMES_HOME", raising=False)
+        module = self._load_helper(monkeypatch)
+        assert module.display_hermes_home() == "~/.hermes"
+
+    def test_fallback_display_hermes_home_profile_path(self, monkeypatch):
+        """Fallback display_hermes_home() handles profile paths under ~/."""
+        monkeypatch.setenv("HERMES_HOME", str(Path.home() / ".hermes/profiles/coder"))
+        module = self._load_helper(monkeypatch)
+        assert module.display_hermes_home() == "~/.hermes/profiles/coder"
+
+    def test_fallback_display_hermes_home_custom_path(self, monkeypatch):
+        """Fallback display_hermes_home() returns full path for non-home locations."""
+        monkeypatch.setenv("HERMES_HOME", "/opt/hermes-custom")
+        module = self._load_helper(monkeypatch)
+        assert module.display_hermes_home() == "/opt/hermes-custom"
+
+    def test_delegates_to_hermes_constants_when_available(self):
+        """When hermes_constants IS importable, _hermes_home delegates to it."""
+        spec = importlib.util.spec_from_file_location(
+            "_hermes_home_happy", self.HELPER_PATH
+        )
+        module = importlib.util.module_from_spec(spec)
+        assert spec.loader is not None
+        spec.loader.exec_module(module)
+        import hermes_constants
+        assert module.get_hermes_home is hermes_constants.get_hermes_home
+        assert module.display_hermes_home is hermes_constants.display_hermes_home