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/workflow-steps.md

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+# Workflow Steps
+
+[← Docs index](./README.md)
+
+Workflow steps are reusable quality gates that run around task completion.
+
+## What They Are
+
+A workflow step is a reusable check (AI prompt or script) that can be enabled on tasks.
+
+Common use cases:
+
+- Documentation review
+- QA/test verification
+- Security scanning
+- Performance checks
+- Accessibility checks
+- Browser-level verification
+
+## Execution Phases
+
+Workflow steps run in one of two phases:
+
+- **Pre-merge** (default): runs before merge/finalization; failure blocks completion
+- **Post-merge**: runs after successful merge; failure is logged but non-blocking
+
+## Execution Modes
+
+- **Prompt mode**: starts an AI agent for the step
+- **Script mode**: runs a named script from project settings (`settings.scripts`)
+
+Prompt mode can run with readonly or coding-capable tool access depending on step/template configuration.
+
+## Built-In Templates (6)
+
+Fusion ships six templates:
+
+1. Documentation Review
+2. QA Check
+3. Security Audit
+4. Performance Review
+5. Accessibility Check
+6. Browser Verification
+
+The Browser Verification template uses browser automation style checks and is designed for UI validation flows.
+
+## Model Overrides for Prompt Steps
+
+A prompt-mode workflow step can specify its own model with:
+
+- `modelProvider`
+- `modelId`
+
+If both are set, step execution uses that model; otherwise it falls back to default model selection.
+
+## Default-On Behavior for New Tasks
+
+Workflow step definitions support `defaultOn`.
+
+When `defaultOn: true`, the step is preselected automatically for newly created tasks (users can still deselect it).
+
+## Viewing Results
+
+Task detail modal includes a **Workflow** tab when workflow data exists.
+
+You can inspect:
+
+- pass/fail/skipped status
+- outputs/findings
+- timing metadata
+
+## Workflow Step APIs
+
+| Endpoint | Purpose |
+|---|---|
+| `GET /api/workflow-steps` | List workflow steps |
+| `POST /api/workflow-steps` | Create workflow step |
+| `PATCH /api/workflow-steps/:id` | Update step |
+| `DELETE /api/workflow-steps/:id` | Delete step |
+| `POST /api/workflow-steps/:id/refine` | AI-refine prompt |
+| `GET /api/workflow-step-templates` | List built-in templates |
+| `POST /api/workflow-step-templates/:id/create` | Materialize template as workflow step |
+
+## Screenshot
+
+![Workflow step manager](./screenshots/workflow-steps.png)
+
+See also: [Task Management](./task-management.md) and [Settings Reference](./settings-reference.md).