Add a token-free, curated affection matcher (agent/reactions.py) — the single
source of truth for detecting user "vibes" (ily / <3 / good bot / heart emoji).
No model call, no tokens. Generalized to return a reaction *kind* so future
reactions can ride the same signal.
Wire an opt-in AIAgent.reaction_callback that fires from build_turn_context on
the incoming user message. It never touches the conversation (cache-safe) and
never fatal — a purely cosmetic side-beat each host can consume.
