feat(webhooks): Real-Time Webhooks & Server-Sent Events (SSE) Streamer

[sdageltc]

Summary & Motivation

For teams embedding letitloop into continuous delivery pipelines, Slack notifications, Discord bots, or custom web dashboards, polling state.json on disk is inefficient.

Adding a lightweight Webhook & SSE Event Streamer will allow external systems to subscribe to real-time execution telemetry.

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Proposed Features

1. Configurable Webhook Dispatcher (orchestrator/webhooks.py):
   • Supports outgoing HTTP POST webhooks to user-configured endpoints (e.g. Slack incoming webhooks, GitHub commit status API, custom server).
   • Event types: goal.started, contract.working, contract.verified, contract.qc_passed, contract.failed, impossibility.generated, goal.completed.
2. Lightweight SSE Server (lil serve --port 8080):
   • Zero-dependency stdlib http.server streaming Server-Sent Events (text/event-stream) for browser-based monitoring tools.
3. Payload Signature Verification:
   • HMAC-SHA256 signature header (X-LetItLoop-Signature) for secure webhook payload validation.

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Acceptance Criteria

• Asynchronous non-blocking HTTP webhook delivery (does not slow down supervisor loop).
• HMAC signature verification for webhook receivers.
• Unit tests for webhook dispatch and payload serialization.