Add reply_to_mode setting (off/first/all) to control whether Telegram
replies quote/thread to the user's original message.
- 'off': Never thread replies (no quote bubble)
- 'first': Only first chunk threads to user's message (default, preserves existing behavior)
- 'all': All chunks in multi-part replies thread to user's message
Configurable via:
- reply_to_mode in platform config (gateway config YAML)
- TELEGRAM_REPLY_TO_MODE env var
Based on PR #855 by raulvidis.
