feat: add design phase and platform app generation phase

Add a UI/UX design phase (/define-design, /update-design) and a platform
app generation phase (/generate-app web|flutter|winui3) to the workflow,
positioned between /setup-project and /add-feature.

- commands: define-design, generate-app, update-design
- add-feature: check docs/design/ before implementation; update design
  files first when a feature changes the UI
- skills: ui-design, design-tokens, platform-ui-generation
- docs/design/ structure documented (source of truth = ui-blueprint.json;
  SVGs are references only; generated UI follows design-tokens.json)
- README and AGENTS.md updated with the new workflow

docs/design/README.md

@@ -0,0 +1,62 @@
+# Design Specification (`docs/design/`)
+
+This directory holds the project's **UI/UX design specification**. It is created and maintained by the `/define-design` and `/update-design` commands and consumed by `/generate-app` and `/add-feature`.
+
+## Source of truth
+
+```
+docs/design/ui-blueprint.json   ← THE source of truth for UI generation
+```
+
+SVG files are **visual references only**. If an SVG and the blueprint disagree, the blueprint wins and the SVG must be regenerated.
+
+Generated UI must follow `design-tokens.json` — no hard-coded values.
+
+## Structure
+
+```
+docs/design/
+├── design-brief.md         # UI/UX direction (human-readable, concrete)
+├── design-tokens.json      # Styling values (color, type, spacing, radius, …)
+├── ui-blueprint.json        # Source of truth: screens, components, routes, actions
+├── platform-mapping.md      # How the design maps to Web / Flutter / WinUI 3
+└── screens/                 # SVG wireframes (one per blueprint screen)
+    ├── home.svg
+    ├── settings.svg
+    └── [screen-id].svg
+```
+
+## File purposes
+
+### `design-brief.md`
+Overall UI/UX direction: target users, design concept, visual mood, layout/navigation principles, accessibility, platform-specific notes, preferred styles, and styles to avoid.
+
+### `design-tokens.json`
+Design tokens in a Design Tokens Community Group–inspired JSON format (`$type` / `$value`). Covers color, typography, spacing, radius, shadow, motion, and breakpoints. Platform code reads from these tokens and never hard-codes values.
+
+### `ui-blueprint.json`
+The source of truth for UI generation. Describes the app name, target platforms, routes, screens (each with a layout and component tree), reusable components (variants and states), and user actions/navigation.
+
+### `platform-mapping.md`
+Documents how `design-tokens.json` and `ui-blueprint.json` are converted to each target platform:
+
+- **Web**: tokens → CSS variables / Tailwind theme; blueprint → pages, routes, components.
+- **Flutter**: tokens → `ThemeData` / `ColorScheme` / constants; blueprint → widgets, routes.
+- **WinUI 3**: tokens → `ResourceDictionary`; blueprint → XAML pages, `UserControl`s, navigation.
+
+### `screens/*.svg`
+Low-fidelity wireframes, one per screen in `ui-blueprint.json`, named `<screen-id>.svg`. Generated for human review only — never the source of truth.
+
+## Workflow
+
+```
+/setup-project   → product/architecture docs
+/define-design   → creates the files above
+/generate-app web | flutter | winui3   → scaffolds the app from these files
+/update-design   → updates these files when a feature changes the UI
+/add-feature     → reads these files before implementing; updates them first if the UI changes
+```
+
+## Status
+
+This directory is populated by running `/define-design`. Until then, the files above do not exist and `/generate-app` will ask you to run `/define-design` first.