Fix UnicodeEncodeError for Cyrillic in email headers ...

_enc_header() checks if a value is ASCII-safe; if not, wraps it in
email.header.Header with utf-8 charset, producing RFC 2047 encoding
(=?utf-8?b?...?=). Applied to Subject and To in _build_mime — covers
both send and reply with Cyrillic subjects or display names.

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

Fix gmail auth: read credentials from settings, not os.environ ...

pydantic-settings loads .env only into the Settings object — it does not
populate os.environ. Added gmail_address and gmail_app_password fields to
Settings; gmail tool now reads from settings instead of os.environ.

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

Rewrite Gmail tool: IMAP/SMTP + App Password (drop OAuth2) ...

Standard library only (imaplib, smtplib) — no google-api dependencies.
Auth via GMAIL_ADDRESS + GMAIL_APP_PASSWORD env vars.
Pagination via integer offset instead of page token.
Message IDs are IMAP UIDs (shown in list output).
html2text still used for HTML→plain conversion on read.

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

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>