Add a Vercel deployment plugin

## Summary
Only Docker is implemented for deployment. Add Vercel output (`vercel.json` + docs) for frontend/Next stacks.

## Tasks
- [ ] `plugins/deployment/vercel.js`
- [ ] Generate `vercel.json` tuned to the selected frontend
- [ ] Register in `src/core/plugin-loader.ts` and `src/core/catalog.ts`

## Files
- `plugins/deployment/vercel.js`
- `src/core/catalog.ts`

Start in `src/core/plugin-loader.ts` and `src/core/catalog.ts` to mirror the existing deployment plugin registration flow, then add `plugins/deployment/vercel.js` for Vercel output generation. Generate/update the `vercel.json` template for frontend/Next stacks and wire the new plugin into the same loading/catalog path. Run the CLI generation flow for a frontend/Next option to confirm the Vercel deployment choice appears and writes the expected config and docs output.

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