Add notify-cascade to Notifications and Messages

[kai-learner]

What is notify-cascade?

notify-cascade is a GitHub Action that sends notifications to Slack, email (SMTP), and custom HTTP webhooks — all in one step, fired in parallel.

Why it's a good addition

Most teams need to notify 2-3 channels on CI events. Today they chain separate actions for each one. notify-cascade handles all three in a single step with a simple YAML config:

- uses: kai-learner/notify-cascade@v1
  with:
    message: 'Deploy to production succeeded 🚀'
    slack-webhook: ${{ secrets.SLACK_WEBHOOK }}
    email-to: team@example.com
    smtp-host: smtp.gmail.com
    smtp-user: ${{ secrets.SMTP_USER }}
    smtp-password: ${{ secrets.SMTP_PASSWORD }}
    webhook-url: ${{ secrets.OPS_WEBHOOK }}

• ✅ All channels fire in parallel (non-blocking)
• ✅ Per-channel status outputs (sent / skipped / failed)
• ✅ Rich Slack block-kit messages with repo/run context
• ✅ HTML + plain-text email, customisable subject
• ✅ Custom webhook headers and body templates with {{message}}, {{repository}}, etc.
• ✅ 10 unit tests, CI on Node 20 + 22, MIT license, node20 runtime

It fits under Notifications and Messages alongside the existing Slack and email actions.