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()

cogs/reactions.py

@@ -0,0 +1,69 @@
+import asyncio
+import logging
+import random
+import time
+
+import discord
+from discord.ext import commands
+
+logger = logging.getLogger("bcs.reactions")
+
+
+class ReactionCog(commands.Cog):
+    def __init__(self, bot: commands.Bot):
+        self.bot = bot
+        # Per-channel timestamp of last reaction
+        self._last_reaction: dict[int, float] = {}
+
+    @commands.Cog.listener()
+    async def on_message(self, message: discord.Message):
+        if message.author.bot or not message.guild:
+            return
+
+        cfg = self.bot.config.get("reactions", {})
+        if not cfg.get("enabled", False):
+            return
+
+        # Skip empty messages
+        if not message.content or not message.content.strip():
+            return
+
+        # RNG gate
+        chance = cfg.get("chance", 0.15)
+        if random.random() > chance:
+            return
+
+        # Per-channel cooldown
+        ch_id = message.channel.id
+        cooldown = cfg.get("cooldown_seconds", 45)
+        now = time.monotonic()
+        if now - self._last_reaction.get(ch_id, 0) < cooldown:
+            return
+
+        # Fire and forget so we don't block anything
+        asyncio.create_task(self._try_react(message, ch_id, now))
+
+    async def _try_react(self, message: discord.Message, ch_id: int, now: float):
+        try:
+            emoji = await self.bot.llm.pick_reaction(
+                message.content, message.channel.name,
+            )
+            if not emoji:
+                return
+
+            await message.add_reaction(emoji)
+            self._last_reaction[ch_id] = now
+            logger.info(
+                "Reacted %s to %s in #%s: %s",
+                emoji, message.author.display_name,
+                message.channel.name, message.content[:60],
+            )
+        except discord.HTTPException as e:
+            # Invalid emoji or missing permissions — silently skip
+            logger.debug("Reaction failed: %s", e)
+        except Exception:
+            logger.exception("Unexpected reaction error")
+
+
+async def setup(bot: commands.Bot):
+    await bot.add_cog(ReactionCog(bot))

config.yaml

@@ -161,3 +161,8 @@ coherence:
     mobile_keyboard: "{username}'s thumbs are having a rough day."
     language_barrier: "Having trouble there, {username}? Take your time."
     default: "You okay there, {username}? That message was... something."
+
+reactions:
+  enabled: true
+  chance: 0.15          # Probability of evaluating a message for reaction
+  cooldown_seconds: 45  # Per-channel cooldown between reactions