docs(windows): document preview installation

[liugddx]

English

Document the unsigned Windows x64 preview without advertising Windows as a supported release tier.

• add SHA-256 verification and SmartScreen guidance;
• document model/ripgrep setup and uninstall entry point;
• state the evidence already provided by the pinned v0.1.9 -> candidate release gate;
• explicitly exclude automatic updates, running-app upgrades, persisted business-data migration, mid-install rollback, signing, sandboxing, and Computer Use.

Validation: npm run format:check, git diff --check.

Tracks the Phase 3 publication/documentation item in #2142.

中文

补充未签名 Windows x64 预览版文档，同时不把 Windows 提前宣传为正式支持的平台。

• 增加 SHA-256 校验和 SmartScreen 操作说明；
• 说明模型、ripgrep 配置和卸载入口；
• 写明固定 v0.1.9 -> 候选版本发布门禁已经提供的证据；
• 明确排除自动更新、运行中升级、业务数据迁移、安装中途 rollback、签名、沙箱和 Computer Use。

验证：npm run format:check、git diff --check。

关联 #2142 Phase 3 发布/安装文档任务。