feat: add ReactionCog for ambient emoji reactions

Add a new cog that gives the bot ambient presence by reacting to
messages with contextual emoji chosen by the triage LLM. Includes
RNG gating and per-channel cooldown to keep reactions sparse and
natural.

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

bot.py

@@ -139,6 +139,7 @@ class BCSBot(commands.Bot):
         await self.load_extension("cogs.sentiment")
         await self.load_extension("cogs.commands")
         await self.load_extension("cogs.chat")
+        await self.load_extension("cogs.reactions")
 
         # Global sync as fallback; guild-specific sync happens in on_ready
         await self.tree.sync()