[Task] Add a PowerShell tamper-detection demo for Windows

[ryan-wong-coder]

Background

scripts/demo.sh gives macOS and Linux users a one-command local demonstration. Windows users need an equivalent PowerShell entry point.

Scope

Add scripts/demo.ps1 that builds the CLI in a disposable directory, creates fresh client and server keys, proves a sample file, verifies the original, modifies a copy, and confirms verification fails.

Acceptance criteria

• Uses a fresh temporary directory and cleans it up.
• Does not overwrite repository or user keys.
• Exits non-zero if modified content is unexpectedly accepted.
• Produces short, understandable progress output.
• README and README.zh-CN mention the Windows command.
• The script is manually validated on Windows PowerShell.