docs: add a coding-agent guide for Vite+ command usage

## Summary

Follow up on recent AI-assisted Vite+ usage: we should document more clearly how coding agents should invoke Vite+ commands, especially when forwarding args to scaffolding/templates.

## Problem

We saw an AI workflow pass extra args to Vite+ incorrectly.

Today the docs mention `vp create -- ` and include a few migration-agent notes, but there is no single page we can point coding agents to for Vite+ command conventions and common gotchas.

## Proposed docs work

- Add a dedicated docs page for coding agents / AI-assisted Vite+ workflows
- Explain when to use `--` to forward args to templates or underlying tools
- Show a few canonical examples, e.g.:
- `vp create vite -- --template react-ts`
- other representative forwarding cases if we support them
- Call out common mistakes explicitly (for example: passing template args directly to `vp create` without `--`)
- Link to this page from the existing create/migrate/agent-related docs so Codex and other agents have an obvious source of truth

## Acceptance criteria

- A dedicated docs page exists and is linked from the relevant Vite+ docs
- The page includes concrete command examples and a short “common mistakes” section
- The guidance is clear enough that we can point coding agents to it directly

## Notes

This is intended as a docs follow-up, not a product behavior change.

Lies zunächst die vorhandene Vite+-Dokumentation zu create, migrate und agent-bezogenen Themen, um die aktuellen Konventionen für Befehle und das Weiterleitungsverhalten zu bestätigen. Füge einen eigenen Leitfaden für Coding Agents mit kanonischen Beispielen, einem Abschnitt zu häufigen Fehlern und Verweisen aus den relevanten Dokumentationen hinzu; abgeschlossen ist die Aufgabe, wenn die Akzeptanzkriterien abgedeckt und die Hinweise klar genug sind, um sie direkt mit Agents zu teilen.

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