feat(vue): add v-model usage example

[axiaowei]

Summary / 摘要

为 @floatboat/nexus-vue 补充 v-model 双向绑定的使用示例，展示 <Editor /> 组件如何与外部状态同步 Markdown 内容。

Motivation / 背景与动机

• Issue: N/A
• Roadmap (docs/ROADMAP.md): P0 — Core API completeness
• OpenSpec change: N/A

当前 README 的 Vue 3 示例仅展示了单向的 initial-value 用法，未体现 v-model 双向绑定能力。而 packages/vue/src/editor.ts 已实现 modelValue prop 与 update:modelValue 事件，缺少对应的使用示例。补充此示例有助于使用者理解受控模式的用法。

Changes / 变更内容

• packages/vue：无代码改动
• docs/examples/vue3-demo/App.vue：新增，Vue 3 v-model 完整示例
• docs/examples/vue3-demo/README.md：新增，示例说明文档

Testing / 测试

• 纯文档示例，不影响构建与测试
• 已确认示例代码与 packages/vue/src/types.ts 中 EditorProps 的 modelValue / onChange 定义一致
• 已在本地运行 pnpm dev:electron-demo，确认 <Editor /> 组件可用

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