Webhook notifications on agent completion

Add webhook notifications from agentd to Ava on agent completion.

Current state:

• Daemon.hs has webhook_url field in SpawnRequest
• pi_rpc.py sends webhooks but we're removing it
• No actual webhook implementation in Daemon.hs

Target state:

• agentd sends HTTP POST to webhook URL on:
• Agent started
• Agent completed (success)
• Agent failed (error)
• Agent stopped (manual)
• Payload includes: run_id, status, summary, cost, duration

Implementation: 1. Add webhook sending in monitorPiAgent when process exits 2. Parse agent output for summary/cost info 3. HTTP POST with JSON payload 4. Retry logic for transient failures 5. Timeout to avoid blocking

Webhook payload:

{
  "run_id": "pi-abc123",
  "event": "completed",
  "status": "success",
  "summary": "Fixed the bug in Foo.hs",
  "cost_cents": 42,
  "duration_seconds": 120,
  "timestamp": "2026-02-03T19:00:00Z"
}