Messaging adapter rate limiting infrastructure

[kovtcharov]

Summary

Per-user, per-channel, and global rate limiting for messaging platform adapters. Prevents abuse and resource exhaustion when GAIA is exposed via Discord/Slack/Telegram.

Configuration

rate_limits:
  user_rpm: 10        # Requests per minute per user
  channel_rpm: 30     # Per channel
  global_rpm: 100     # Total
  cooldown_seconds: 5 # Min time between same-user requests

Acceptance Criteria

• Token bucket rate limiter with configurable limits
• Per-user, per-channel, and global tiers
• Polite decline message when rate limited (not silent drop)
• Configurable via messaging.yaml
• Rate limit state persists across adapter restarts

Dependencies

• Messaging adapters: Telegram, Discord, Slack integration #635 (messaging adapters)