docs(web): add AGENTS.md — stack, commands, routing conventions, migration rules

apps/web/AGENTS.md

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+# Web App — Agent Instructions
+
+Applies to all code under `apps/web/`. Subordinate to [`apps/AGENTS.md`](../AGENTS.md) and root [`AGENTS.md`](../../AGENTS.md).
+
+## Conventions and style
+
+Read these before making changes:
+
+- **[`CONVENTIONS.md`](./CONVENTIONS.md)** — Architecture, code organization, state management, component patterns, framework strategy, data fetching, testing.
+- **[`STYLE_GUIDE.md`](./STYLE_GUIDE.md)** — Naming, imports, TypeScript, component authoring, formatting.
+
+## Stack
+
+- **Build**: [Vite](https://vite.dev/) + [React 19](https://react.dev/blog/2024/12/05/react-19)
+- **Routing**: [React Router v7](https://reactrouter.com/) — [data mode](https://reactrouter.com/start/modes) (`createBrowserRouter`), NOT framework mode
+- **Client state**: [Zustand](https://zustand.docs.pmnd.rs/) (migration in progress from `useReducer` + Context)
+- **Server state**: [TanStack Query](https://tanstack.com/query/latest) with [HeyAPI plugin](https://heyapi.dev/openapi-ts/plugins/tanstack-query)
+- **Styling**: [Tailwind CSS v4](https://tailwindcss.com/) via `@tailwindcss/vite`
+- **Design system**: `@vellum/design-library` at [`packages/design-library/`](../../packages/design-library/)
+- **Platform**: Web + iOS via [Capacitor](https://capacitorjs.com/) — native code paths must be preserved
+
+## Routing
+
+- Route config: `src/routes.tsx`
+- Route constants: `src/utils/routes.ts` — all paths are absolute browser paths
+- No `basename` on the router — `/account/*` and `/assistant/*` are explicit top-level branches
+- Routes must match the platform repo exactly during migration (no URL changes)
+
+## Commands
+
+```bash
+cd apps/web && bun install            # Install dependencies
+cd apps/web && bun run dev            # Vite dev server (port 3001)
+cd apps/web && bun run openapi-ts     # Generate API client from OpenAPI specs
+cd apps/web && bunx tsc --noEmit      # Type-check
+cd apps/web && bun run lint           # Lint
+cd apps/web && bun run build          # Production build
+cd apps/web && bun test src/path/to/file.test.ts  # Run specific tests
+```
+
+## Migration status
+
+This app is being migrated from [`vellum-assistant-platform/web/`](https://github.com/vellum-ai/vellum-assistant-platform). During migration:
+
+- **Faithful copy, not simplification.** Port real implementations, not stubs. All Capacitor/native code paths must be preserved.
+- **Convention compliance on arrival.** Apply this repo's naming (kebab-case), import conventions (`.js` extensions, `@/` aliases), and directory structure as code is ported.
+- **No marketing or admin pages.** Only the assistant web app and auth/identity pages are migrating.