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/multi-project.md

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+# Multi-Project
+
+[← Docs index](./README.md)
+
+Fusion can coordinate multiple repositories from one installation, with shared visibility and global concurrency control.
+
+## Why Use Multi-Project Mode?
+
+Use multi-project mode when you need to:
+
+- Operate many repos from one dashboard/CLI
+- Standardize settings and workflows across projects
+- Monitor global activity and system-wide execution capacity
+
+## Central Database Architecture
+
+Multi-project metadata is stored in:
+
+`~/.pi/fusion/fusion-central.db`
+
+Core tables:
+
+- `projects`
+- `projectHealth`
+- `centralActivityLog`
+- `globalConcurrency`
+
+Per-project task data remains in each repo’s `.fusion/fusion.db`.
+
+## Registering and Managing Projects
+
+```bash
+fn project add my-app /path/to/app
+fn project list
+fn project show my-app
+fn project set-default my-app
+fn project detect
+fn project remove my-app --force
+```
+
+## `--project` Flag and Resolution
+
+You can target a project explicitly:
+
+```bash
+fn task list --project my-app
+fn task create "Fix oauth callback" --project my-app
+```
+
+Resolution order without `--project`:
+
+1. explicit flag
+2. default project
+3. current-directory auto-detection
+
+## Project Health Tracking
+
+Central health tracking keeps mutable project metrics, including:
+
+- active task counts
+- in-flight agent counts
+- project status (`initializing`, `active`, `paused`, `errored`)
+
+## Global Concurrency Management
+
+A singleton central record enforces system-wide limits so one project cannot monopolize all execution slots.
+
+## Isolation Modes
+
+Projects can run with:
+
+- **`in-process`** (default): low overhead, shared process
+- **`child-process`**: stronger isolation with independent process boundary
+
+## Auto-Migration from Single-Project
+
+On first run after upgrade:
+
+- Existing project databases are detected
+- Projects are registered into central DB automatically
+- Existing single-project workflows continue working
+
+Migration is idempotent and designed to avoid repeated re-registration.
+
+## Rollback Procedure
+
+If central registry behavior needs to be reverted:
+
+1. Delete `~/.pi/fusion/fusion-central.db`
+2. Keep using per-project `.fusion/fusion.db` data
+3. Fusion falls back to legacy/single-project behavior
+4. Re-register projects later with `fn init` / `fn project add`
+
+## Runtime Architecture
+
+### ProjectRuntime interface
+
+Each project runtime supports start/stop/status/metrics and access to scheduler/task store (for in-process mode).
+
+### HybridExecutor
+
+HybridExecutor orchestrates all project runtimes and forwards project-attributed events.
+
+### IPC Protocol (child-process mode)
+
+Host → worker commands include:
+
+- `START_RUNTIME`
+- `STOP_RUNTIME`
+- `GET_STATUS`
+- `GET_METRICS`
+- `PING`
+
+Worker → host events include:
+
+- `TASK_CREATED`
+- `TASK_MOVED`
+- `TASK_UPDATED`
+- `ERROR_EVENT`
+- `HEALTH_CHANGED`
+
+## HybridExecutor Diagram
+
+```mermaid
+flowchart TD
+    HE[HybridExecutor]
+    PM[Project Manager]
+    CC[CentralCore]
+
+    HE --> PM
+    HE --> CC
+
+    PM --> A[Project A Runtime\n(in-process)]
+    PM --> B[Project B Runtime\n(child-process)]
+    PM --> C[Project C Runtime\n(in-process)]
+
+    B --> IPC[IPC Worker Channel]
+```
+
+See also: [Architecture](./architecture.md), [CLI Reference](./cli-reference.md), and [Missions](./missions.md).