docs(design): add Knowledge Module design and implementation plan

- Full design document with architecture, data model, API specs
- 28 implementation issues across 5 phases (~127h total)
- Wiki-link syntax, semantic search, graph visualization
- Integration points for agent access

Ref: memory/2025-01-29-agent-orchestration.md

docs/design/README.md

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+# Design Documents
+
+Technical design documents for major Mosaic Stack features.
+
+## Purpose
+
+Design documents serve as:
+- **Blueprints** for implementation
+- **Reference** for architectural decisions
+- **Communication** between team members
+- **Historical record** of design evolution
+
+## Document Structure
+
+Each design document should include:
+
+1. **Problem Statement** — What are we solving?
+2. **Architecture Overview** — High-level design with diagrams
+3. **Database Schema** — Tables, indexes, relationships
+4. **API Specifications** — Endpoints, request/response formats
+5. **Implementation Plan** — Phased rollout with milestones
+6. **Security & Performance** — Considerations and constraints
+
+## Documents
+
+### [Agent Orchestration Layer](./agent-orchestration.md)
+
+**Status:** Design Phase  
+**Version:** 1.0  
+**Date:** 2025-01-29
+
+Infrastructure for persistent task management and autonomous agent coordination. Enables long-running background work independent of user sessions.
+
+**Key Features:**
+- Task queue with priority scheduling
+- Agent health monitoring and automatic recovery
+- Checkpoint-based resumption for interrupted work
+- Multi-workspace coordination with row-level security
+
+---
+
+### [Knowledge Module](./knowledge-module.md)
+
+**Status:** Design Phase  
+**Version:** 1.0  
+**Date:** 2025-01-29  
+**Issues:** [Implementation Tracker](./knowledge-module-issues.md)
+
+Native knowledge management with wiki-style linking, semantic search, and graph visualization. Enables teams and agents to capture, connect, and query organizational knowledge.
+
+**Key Features:**
+- Wiki-style `[[links]]` between entries
+- Full-text and semantic (vector) search
+- Interactive knowledge graph visualization
+- Version history with diff view
+- Tag-based organization
+
+---
+
+## Contributing
+
+When creating a new design document:
+
+1. Copy the structure from an existing document
+2. Use ASCII diagrams for architecture (keep them simple)
+3. Include code examples in TypeScript
+4. Specify database schema in SQL (PostgreSQL dialect)
+5. Add implementation phases with clear milestones
+6. Update this README with a summary
+
+---
+
+**Last Updated:** 2025-01-29