Add versioning schema: version (int), version_string, version_release_date to all 10 connectors

- version: intero per-connector (parte da 1)
- version_string: semver ereditato (display only)
- version_release_date: ISO 8601 (display only)
- Campi identici in index (connectors.json) e manifest (connector.json)
- Aggiunto connector.json alle files[] di gmail, gcal, email, ssh, tavily
- Aggiornata documentazione in SKALD.md

connectors/gmail/connector.json

@@ -1,7 +1,6 @@
 {
   "id": "gmail",
   "name": "Gmail",
-  "version": "1.0.0",
   "type": "mcp_local",
   "launch_command": "python3 gmail_mcp_server.py",
   "transport": "stdio",
@@ -51,5 +50,8 @@
       "format": "google_authorized_user",
       "env": "GMAIL_CREDS_JSON"
     }
-  }
-}
+  },
+  "version": 1,
+  "version_string": "1.0.0",
+  "version_release_date": "2026-07-19"
+}