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.