Switch Gmail and Gcal to env-var OAuth delivery pattern

auth.deliver changes from as:file to as:env:
- Gmail: GMAIL_CREDS_JSON env var (Google authorized_user JSON)
- Gcal: GCAL_CREDS_JSON env var

mcp_config.env removed entirely — Skald injects the env var at runtime,
no path on disk needed.

Server scripts updated:
- Check GMAIL_CREDS_JSON / GCAL_CREDS_JSON env var first
- Use Credentials.from_authorized_user_info() instead of
  from_authorized_user_file()
- Fall back to file-based loading for standalone/legacy use
- _persist_creds only writes to disk when _creds_path is set

gcal/verify.py: support GCAL_CREDS_JSON env var with shared _check_api()

Docs (SKALD.md): updated examples, field table, connector table.

connectors/gmail/connector.json

@@ -16,9 +16,7 @@
   ],
   "setup_instructions": [
     "Install dependencies: pip install -r requirements.txt",
-    "Create secrets/google_oauth_client.json with {\"client_id\": \"...\", \"client_secret\": \"...\"}",
-    "Run: python3 gmail_oauth_setup.py (opens browser for OAuth)",
-    "Set GMAIL_CREDS_PATH env var or place token at secrets/gmail_creds.json"
+    "Run: python3 gmail_oauth_setup.py (optional, for standalone use — Skald handles OAuth)"
   ],
   "docs": [
     {
@@ -31,10 +29,7 @@
     "command": "python3",
     "args": [
       "gmail_mcp_server.py"
-    ],
-    "env": {
-      "GMAIL_CREDS_PATH": "{secrets}/gmail_creds.json"
-    }
+    ]
   },
   "homepage": "https://mail.google.com",
   "icon_small": "icon_sm.svg",
@@ -54,9 +49,9 @@
       "https://www.googleapis.com/auth/gmail.labels"
     ],
     "deliver": {
-      "as": "file",
+      "as": "env",
       "format": "google_authorized_user",
-      "path": "{secrets}/gmail_creds.json"
+      "env": "GMAIL_CREDS_JSON"
     }
   }
 }