Add Slack integration for homework PR notification #11

Feature Request: Slack Integration for Homework PRs

Problem

Currently, students and instructors don't get real-time notifications when homework PRs change
status. Students have to manually check GitHub to see if their homework was accepted or needs
changes, and instructors have to actively monitor the repository for stuck or problematic PRs.

Proposed Solution

Add Slack integration to automatically notify students and instructors when homework PRs chang
e status:

• When a PR is opened (new submission)
• When a PR passes all checks and is merged
• When a PR is flagged for issues
• When a PR is closed (stale)
• When a PR has been open for an extended period

Benefits

• For students: Immediate feedback on homework status without checking GitHub
• For instructors: Automated monitoring of PR status and issues
• Better communication: Real-time updates in the existing Slack workspace
• Non-intrusive: Optional feature that can be enabled/disabled easily

Implementation

• Integrate into existing PR bot workflow
• Support both webhook and bot token authentication
• Provide comprehensive setup documentation
• Enable/disable via GitHub Secrets

Alternatives Considered

• Email notifications (more complex setup, less immediate)
• GitHub's built-in notifications (not real-time, requires GitHub monitoring)