Purge standalone OAuth setup for Gmail and Gcal

- Deleted gmail_oauth_setup.py and gcal_oauth_setup.py (Skald handles OAuth)
- Removed google-auth-oauthlib dependency from requirements.txt and connector.json
- Simplified setup_instructions to point to Skald's OAuth flow
- Updated docstrings and error messages in *_mcp_server.py (no more refs to
  *_oauth_setup.py, point to Skald OAuth / env-var instead)
- connectors.json: removed oauth_setup.py from files[], updated sha256 hashes

connectors/gmail/connector.json

@@ -11,12 +11,10 @@
   ],
   "dependencies": [
     "google-api-python-client>=2.150.0",
-    "google-auth>=2.35.0",
-    "google-auth-oauthlib>=1.2.0"
+    "google-auth>=2.35.0"
   ],
   "setup_instructions": [
-    "Install dependencies: pip install -r requirements.txt",
-    "Run: python3 gmail_oauth_setup.py (optional, for standalone use — Skald handles OAuth)"
+    "Activated from Skald: an admin configures the Google sign-in provider, then each user signs in from the connector page (OAuth handled by Skald)."
   ],
   "docs": [
     {