Add Slack notification provider alongside Telegram

[mataeil]

Currently only Telegram is supported for notifications (config.notifications.telegram).

Slack is where most developers already live. Adding a Slack webhook provider would significantly expand the notification reach.

Expected behavior:

• Add notifications.slack section to config schema with webhook_url field
• Use $OODA_SLACK_WEBHOOK_URL env var reference
• Same 3-consecutive-failure auto-disable logic as Telegram
• Document in SECURITY.md (webhook URLs are secrets)

This is a Tier 1 (Skills) contribution — self-contained, follows existing notification pattern.