Add Gmail tool: send, list, list_unread, read, reply via Gmail API ...

- tools/gmail.py: single tool with 'action' param routing to 5 operations.
  Prefers text/plain MIME part; converts HTML→text via html2text for
  HTML-only emails. Truncates body at 5000 chars. Marks message read on
  'read'. Reply preserves thread via threadId + In-Reply-To/References headers.
  Outgoing emails sent as multipart/alternative (plain + html).
- tools/gmail_auth.py: one-time OAuth2 setup script (InstalledAppFlow).
  Reads gmail_credentials.json, saves token to gmail_token.json.
- tools/enabled.json: add gmail to global tool list
- Dependencies: google-api-python-client, google-auth-oauthlib, html2text

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>