Implement alert delivery channels: Slack, webhook, email

[mighty840]

AlertDeliveryChannels struct exists in config (slack, webhook, email fields) but no delivery code is implemented. Conversational alerts are only visible via orca alerts list.

Implement:

1. Slack — POST to incoming webhook URL with threaded conversation updates
2. Webhook — POST JSON alert payload to a configured URL
3. Email — SMTP delivery for alert digests

Config:

[ai.alerts.channels]
slack = "https://hooks.slack.com/services/..."
webhook = "https://my-app.com/api/alerts"
email = "ops@example.com"