feat: reply-decision gate to stop infinite loops in group chats (#36)

[mattmezza]

Closes #36.

Problem

In a shared group chat that mixes multiple bots and people, an "always reply"
agent can get caught in an infinite reaction loop — bot A replies to bot B
replies to bot A… — and replies to messages plainly aimed at someone else.

What this does

Adds an opt-in reply-decision layer so the agent decides, per message,
whether a reply is warranted at all. Two independent mechanisms:

1. Smart gate (core/reply_decision.py) — a cheap one-shot LLM call
   (REPLY/SKIP) that stays quiet for messages addressed to another bot,
   messages in a bot-to-bot loop, or messages it can't usefully add to. It
   fails open (replies) on any error, so a classifier hiccup never drops a
   real user message.
2. Hard rate cap — a per-chat backstop (max_replies_per_window /
   window_seconds) that guarantees a runaway loop terminates even if the gate
   keeps voting "reply". An LLM-only gate can't make that guarantee on its own.

Where it hooks in

AgentCore.process() is the single chokepoint every channel (Telegram,
WhatsApp, voice, REPL, scheduler) funnels through, so one gate covers them all.
It runs right after persona resolution and returns an empty response on skip, so
no expensive inference or tool loop runs for a suppressed message.

Scope guards

• Off by default — 1:1 chats always reply and the gate costs one extra call.
• Group-only by default — only gates chats where chat_id != user_id
  (Telegram private chats and WhatsApp DMs reuse the user's id as the chat id).
• Never gates scheduler/system turns — proactive jobs are untouched.
• WhatsApp now stays silent on an empty response instead of sending a blank
  message.

Config (reply_decision, off by default)

reply_decision:
  enabled: false
  model: "claude-haiku-4-5"
  group_only: true
  max_replies_per_window: 6
  window_seconds: 120

Tests

tests/test_reply_decision.py covers the classifier (REPLY/SKIP/fail-open),
the group-vs-DM heuristic, the rate-cap backstop (incl. window expiry), and the
end-to-end suppression path through process(). Full suite green
(uv run pytest).

Docs: README feature bullet + configuration.mdx section + config.yml.example.