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/gcal/connector.json

@@ -15,9 +15,7 @@
   ],
   "setup_instructions": [
     "Install dependencies: pip install -r requirements.txt",
-    "Create secrets/google_oauth_client.json with {\"client_id\": \"...\", \"client_secret\": \"...\"}",
-    "Run: python3 gcal_oauth_setup.py (opens browser for OAuth)",
-    "Set GOOGLE_CREDS_PATH env var or place token at secrets/google_creds.json"
+    "Run: python3 gcal_oauth_setup.py (optional, for standalone use — Skald handles OAuth)"
   ],
   "docs": [
     {
@@ -33,17 +31,14 @@
       "https://www.googleapis.com/auth/calendar"
     ],
     "deliver": {
-      "as": "file",
+      "as": "env",
       "format": "google_authorized_user",
-      "path": "{secrets}/google_creds.json"
+      "env": "GCAL_CREDS_JSON"
     }
   },
   "mcp_config": {
     "command": "python3",
-    "args": ["gcal_mcp_server.py"],
-    "env": {
-      "GOOGLE_CREDS_PATH": "{secrets}/google_creds.json"
-    }
+    "args": ["gcal_mcp_server.py"]
   },
   "verify": {
     "command": "python3 verify.py",