feat: dynamic Opus/Sonnet model switching based on rolling quota

Implement intelligent model selection to manage 7-day Opus quota burn rate:

- Add _select_model_based_on_quota() method to AgentLoop
  - Reads rate limit data from memory/rate_limits.json
  - Calculates expected vs actual quota usage (100%/168h = 0.595% per hour)
  - If actual > expected × 1.17 (17% overage), downgrades to Sonnet
  - If actual ≤ expected, uses Opus
  - Caches decision for 5 minutes to minimize file I/O

- Add /quota slash command to display real-time quota status
  - Shows current usage vs expected usage
  - Shows hours until weekly reset
  - Shows selected model and burn rate multiplier

- Main agent now calls _select_model_based_on_quota() before each conversation
  - Heartbeat subagent unaffected (explicitly uses claude-sonnet-4-20250514)

This replaces the wrong approach from PR #5 which throttled heartbeat
frequency instead of switching the main agent's model.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nanobot/channels/telegram.py

@@ -92,6 +92,7 @@ class TelegramChannel(BaseChannel):
         BotCommand("start", "Start the bot"),
         BotCommand("new", "Start a new conversation"),
         BotCommand("help", "Show available commands"),
+        BotCommand("quota", "Show current quota status"),
     ]
     
     def __init__(
@@ -127,6 +128,7 @@ class TelegramChannel(BaseChannel):
         self._app.add_handler(CommandHandler("start", self._on_start))
         self._app.add_handler(CommandHandler("new", self._forward_command))
         self._app.add_handler(CommandHandler("help", self._forward_command))
+        self._app.add_handler(CommandHandler("quota", self._forward_command))
         
         # Add message handler for text, photos, voice, documents
         self._app.add_handler(