stage-12: Telegram bot adapter

- New src/adapters/telegram_bot.py: full Telegram adapter with python-telegram-bot v22
  - Commands: /start, /help, /clear, /status, /model, /register
  - Inline keyboards for model selection
  - Message routing through existing router.py
  - Private chat: admin-only access
  - Group chat: responds to @mentions and replies to bot
  - Security logging for unauthorized access attempts
  - Message splitting for 4096 char limit
- Updated main.py: runs Discord + Telegram bots concurrently
  - Telegram is optional (gracefully skipped if no telegram_token)
- Updated requirements.txt: added python-telegram-bot>=21.0
- Updated config.json: added telegram_channels section
- Updated cli.py doctor: telegram token check (optional)
- 37 new tests (410 total, zero failures)

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

requirements.txt

@@ -1,4 +1,5 @@
 discord.py>=2.3
+python-telegram-bot>=21.0
 apscheduler>=3.10
 keyring>=25.0
 keyrings.alt>=5.0