docs(FN-1158): add comprehensive project documentation suite

- Add docs index with structured navigation and links across the new documentation set
- Document onboarding and technical foundations with getting-started and architecture guides
- Add detailed CLI and settings references plus feature guides for dashboard, tasks, workflow steps, missions, multi-project, and agents
- Add a contributing guide and polish cross-references between docs pages for discoverability
- Include dashboard screenshots for key views to provide visual context in the guides

docs/README.md

@@ -0,0 +1,44 @@
+# Fusion Documentation
+
+[← Back to repository root](../README.md)
+
+Fusion is an AI-orchestrated task board that turns ideas into reviewed, merged code using a structured workflow: **triage → todo → in-progress → in-review → done**.
+
+![Fusion Dashboard Overview](screenshots/dashboard-overview.png)
+
+## Quick Start
+
+Install Fusion globally, run `fn dashboard`, then create your first task from the board or CLI.
+
+For a full walkthrough (installation, onboarding, first task, and lifecycle), start here:
+
+➡️ **[Getting Started](./getting-started.md)**
+
+## Documentation Index
+
+| Guide | What it covers |
+|---|---|
+| [Getting Started](./getting-started.md) | Installation, first-run onboarding, first task, and daily workflow basics. |
+| [Architecture](./architecture.md) | System architecture, package layout, storage model, and engine execution flow. |
+| [CLI Reference](./cli-reference.md) | Complete `fn` command reference with subcommands, flags, and examples. |
+| [Dashboard Guide](./dashboard-guide.md) | Detailed guide to board/list views, terminal, git manager, files, planning, and UI tools. |
+| [Task Management](./task-management.md) | Task creation modes, lifecycle, prompt specs, comments, archiving, and GitHub integration. |
+| [Missions](./missions.md) | Mission hierarchy, planning flow, activation, progress tracking, and autopilot behavior. |
+| [Agents](./agents.md) | Agent management, presets, prompts, heartbeat behavior, spawning, and mailbox workflows. |
+| [Workflow Steps](./workflow-steps.md) | Reusable quality gates, templates, pre/post-merge phases, and workflow execution results. |
+| [Settings Reference](./settings-reference.md) | Global and project settings, defaults, API endpoints, and model selection hierarchy. |
+| [Multi-Project](./multi-project.md) | Central registry architecture, project management, isolation modes, and migration paths. |
+| [Contributing](./contributing.md) | Local development setup, testing, release flow, and contributor conventions. |
+| [Code Signing Setup](./CODE_SIGNING.md) | macOS and Windows code signing configuration for release binaries. |
+
+## External Resources
+
+- GitHub repository: https://github.com/gsxdsm/fusion
+- npm package: https://www.npmjs.com/package/@gsxdsm/fusion
+- pi agent framework: https://github.com/badlogic/pi-mono
+
+## Suggested Reading Paths
+
+- **New user:** Getting Started → Dashboard Guide → Task Management
+- **Power user / automation owner:** Settings Reference → Workflow Steps → Agents
+- **Maintainer / contributor:** Architecture → Multi-Project → Contributing