Add Slack/Discord notification integration

[GodsBoy]

Send notifications to Slack or Discord when RepoKeeper triages an issue or reviews a PR.

Scope:

• Add optional webhook URL config for Slack/Discord
• Post formatted messages on triage events
• Include issue/PR link, classification, and summary

Simple integration, great starter issue.

[GodsBoy]

Thanks for opening this issue! Notification integrations are definitely a valuable feature for RepoKeeper. Before we move forward, I'd like to gather a few more details to make sure we scope this correctly:

• Which platform(s) are you prioritizing? Slack, Discord, or both equally?
• What's your expected workflow? Should notifications be sent for all triage events, or would you want filtering/configuration options?
• Are there specific webhook formats or message templates you'd prefer, or should we design something flexible?
• Does your team have any existing notification infrastructure we should be aware of or integrate with?

A rough mockup or example of what you'd like the notification to look like would be super helpful too! This will help us design an API that works well for your use case.