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>

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