fix(skills): ship google-workspace deps as [google] extra; make setup.py 3.9-parseable

Closes #13626.

Two follow-ups on top of the _hermes_home helper from @jerome-benoit's #12729:

1. Declare a [google] optional extra in pyproject.toml
   (google-api-python-client, google-auth-oauthlib, google-auth-httplib2) and
   include it in [all]. Packagers (Nix flake, Homebrew) now ship the deps by
   default, so `setup.py --check` does not need to shell out to pip at
   runtime — the imports succeed and install_deps() is never reached.
   This fixes the Nix breakage where pip/ensurepip are stripped.

2. Add `from __future__ import annotations` to setup.py so the PEP 604
   `str | None` annotation parses on Python 3.9 (macOS system python).
   Previously system python3 SyntaxError'd before any code ran.

install_deps() error message now also points users at the extra instead of
just the raw pip command.

skills/productivity/google-workspace/scripts/setup.py

@@ -21,6 +21,8 @@ Agent workflow:
   6. Run --check to verify. Done.
 """
 
+from __future__ import annotations  # allow PEP 604 `X | None` on Python 3.9+
+
 import argparse
 import json
 import os
@@ -110,7 +112,11 @@ def install_deps():
         return True
     except subprocess.CalledProcessError as e:
         print(f"ERROR: Failed to install dependencies: {e}")
-        print(f"Try manually: {sys.executable} -m pip install {' '.join(REQUIRED_PACKAGES)}")
+        print(
+            "On environments without pip (e.g. Nix), install the optional extra instead:"
+        )
+        print("  pip install 'hermes-agent[google]'")
+        print(f"Or manually: {sys.executable} -m pip install {' '.join(REQUIRED_PACKAGES)}")
         return False