fix: address review feedback for ReactionCog

- Use time.monotonic() at reaction time instead of stale message-receive timestamp
- Add excluded_channels config and filtering
- Truncate message content to 500 chars in pick_reaction

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

utils/llm_client.py

@@ -782,7 +782,7 @@ class LLMClient:
                     model=self.model,
                     messages=[
                         {"role": "system", "content": prompt},
-                        {"role": "user", "content": f"[#{channel_name}] {message_text}"},
+                        {"role": "user", "content": f"[#{channel_name}] {message_text[:500]}"},
                     ],
                     **temp_kwargs,
                     max_completion_tokens=16,