Implements per-agent backoff tracking to reduce noise for busy agents:
- AgentBackoff type tracks interval, miss count, and last activity
- BackoffManager manages state across all agents
- Geometric backoff strategy (1.5x factor, 10min cap)
- Integrates with keepalive to skip pokes when agents are fresh
- Resets backoff immediately when activity detected
Closes gt-8bx
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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