Issue 20 TanStack Start Wiki

Initial Situation

GitHub issue #20 asks for TanStack Start support in the wiki scaffold. dp scaffold init currently prompts for backlog provider only, then seeds a Fumadocs wiki root at wiki or apps/wiki. The generated wiki template is hard-coded to Next.js: package.json uses next scripts, next.config.mjs, App Router files, fumadocs-ui/provider/next, and Next-specific type generation.

Fumadocs supports TanStack Start as a first-class React framework option in create-fumadocs-app, and its current manual install docs require Vite/TanStack routes, fumadocs-ui/provider/tanstack, @tanstack/react-start, and server route handlers for search. TanStack Start routes live under src/routes.

Debt

The scaffolded wiki has no durable framework choice. If a repo wants a TanStack Start Fumadocs wiki, the operator must hand-convert the generated Next.js app. dp update would then restage the wiki from the same Next-only template and risk converting the wiki back to Next.js.

Bounded Fix

• Add an explicit Next.js vs TanStack Start wiki framework prompt to dp scaffold init, defaulting to Next.js.
• Persist the selected wiki framework in the wiki root beside backlog-provider.md.
• Parameterize the wiki template so Next.js output remains the default and TanStack Start output has equivalent Harness wiki structure: project routes, content sync, Mermaid MDX, source loader, search endpoint, routed project docs, AGENTS.md, and bookkeeping files.
• Teach dp update wiki alignment to read the persisted framework pin and default to Next.js when the pin is absent.
• Keep framework choice local to the wiki scaffold. Do not introduce a broad TanStack Start pack unless implementation proves it is needed beyond wiki generation.
• Update scaffold docs and runbooks so operators know the wiki framework is selected and preserved.

Non-Goals

• Replacing the current Harness repo apps/wiki app with TanStack Start.
• Adding support for Waku or React Router wiki scaffolds.
• Building a generic framework abstraction for every Fumadocs variant.
• Changing backlog-provider behavior.
• Publishing a new CLI release unless explicitly requested after validation.

Acceptance Checks

• Scaffold init tests prove default Next.js output and the wiki-framework.md pin.
• Scaffold init tests prove selecting TanStack Start emits Vite/TanStack/Fumadocs files, dependencies, scripts, search route, docs route, source config, Mermaid support, and the same project content tree.
• Update tests prove an existing TanStack Start wiki remains TanStack Start after dp update.
• Existing Next.js wiki scaffold tests keep passing.
• Docs describe the framework choice and update preservation behavior.
• Focused CLI tests, CLI typecheck, CLI check, and diff validation pass.