Agent-to-agent webhook federation

[christinoleo]

Enable agents to sync session state via webhooks for distributed deployments.

Architecture

• Agent watches local JSON files (as now)
• On change, POST webhook to configured remote agents
• Remote agents update their state and push to connected browsers

Config

{
  "sync_to": ["http://hub-agent:3456/api/webhook"]
}

Acceptance Criteria

• Agent config for remote agent URLs
• Webhook endpoint to receive session updates
• POST to remotes on local file change
• Deduplication (don't echo back received updates)

Parent epic: #1
Depends on: #3 (workspace field)

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Migrated from beads claude-watch-j0k.