Add comprehensive Slack integration design document

[Copilot]

This PR adds a comprehensive design document (SLACK_INTEGRATION.md) that outlines how Slack integration could work within the Morpheum ecosystem. The design maintains Morpheum's core Matrix-based, federated architecture while providing an optional bridge for teams that prefer or require Slack as their collaboration platform.

Key Design Features

Bridge Pattern Architecture: The design uses a bridge service that maintains Morpheum's Matrix-centric core while adding Slack compatibility. This preserves the federated, decentralized principles while expanding platform accessibility.

Full Feature Parity: All existing bot capabilities (multi-LLM support, GitHub integration, project management) would be available through Slack interfaces using slash commands and interactive elements.

Security-First Approach: Comprehensive authentication design using OAuth 2.0, message encryption, and granular access controls that respect both platform's security models.

Phased Implementation: A practical 16-week roadmap broken into four phases:

• Foundation (Weeks 1-4): Core bridge infrastructure
• Core Features (Weeks 5-8): Rich messaging and GitHub integration
• Advanced Integration (Weeks 9-12): Cross-platform synchronization
• Production Readiness (Weeks 13-16): Documentation and migration tools

Technical Highlights

The document includes detailed technical specifications for:

• Message format translation between Slack blocks and Matrix markdown
• Command mapping (!help → /morpheum help) with interactive buttons and modals
• Cross-platform user identity management and room/channel bridging
• Enterprise features like SSO integration and compliance logging

Real-World Examples

Practical usage scenarios demonstrate how teams could:

• Create GitHub issues and PRs through Slack using AI agents
• Switch between LLM providers (OpenAI, Ollama, GitHub Copilot) via slash commands
• Manage development workflows with interactive Slack elements
• Migrate gradually from Matrix while maintaining cross-platform collaboration

The design ensures that Slack integration serves as an accessibility layer rather than a replacement, allowing Morpheum to reach broader developer communities while preserving its architectural integrity.

Fixes #159.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.