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>

config.json

@@ -1,11 +1,17 @@
 {
   "bot": {
     "name": "Echo",
-    "default_model": "sonnet",
-    "owner": null,
+    "default_model": "opus",
+    "owner": "949388626146517022",
     "admins": []
   },
-  "channels": {},
+  "channels": {
+    "echo-core": {
+      "id": "1471916752119009432",
+      "default_model": "opus"
+    }
+  },
+  "telegram_channels": {},
   "heartbeat": {
     "enabled": true,
     "interval_minutes": 30
@@ -20,4 +26,4 @@
     "logs": "logs/",
     "sessions": "sessions/"
   }
-}
+}