install: add Windows (PowerShell) one-liner installer matching scripts/install.sh

[jsyqrt]

Goal

Windows users currently must install via npm or the Electron desktop package. Add a PowerShell installer (iwr -UseBasicParsing ... | iex) that mirrors scripts/install.sh: detect architecture, download the right binary bundle, verify checksum, and print next steps.

Reference files

• scripts/install.sh (existing bash installer — mirror its logic)
• scripts/build-binary.sh (what artifacts exist for Windows)
• markus.global/install.sh endpoint (where the script is served)

Acceptance criteria

• install.ps1 works on PowerShell 5.1+ and 7+ (x64 + arm64)
• Downloads and verifies checksum, installs to %LOCALAPPDATA%\Markus (or similar), adds to PATH
• Documented in README Quick Start (Windows line)
• No secrets; respects standard security (RemoteSigned / execution policy guidance)

Size: M · Skill needed: PowerShell, release engineering · Comment before starting to coordinate with release automation.