[install] Add native Windows install instructions

[mccarthy606]

The current install.sh is a bash script that runs on macOS + Linux. Windows users are pointed to WSL.

A native PowerShell installer (install.ps1) would lower the friction for Windows users who aren't running WSL.

What to do

1. Port the logic of install.sh to install.ps1:
   • Step 1: check for claude CLI in PATH
   • Step 2: print marketplace + plugin instructions
   • Step 3: copy sanitized configs to $env:USERPROFILE\.claude\ as sidecar files
   • Step 4: print next-step checklist
2. Match the existing flags: -DryRun, -Yes
3. Match the existing colorized output (use Write-Host -ForegroundColor)
4. Add a Windows section to README's Quick Start that surfaces the new path
5. Update CHANGELOG.md

Reference

• install.sh is the source-of-truth — port behavior 1:1
• ECC has both install.sh and install.ps1 and uses node scripts/install-apply.js as the unified runtime — that pattern is also fine if you prefer it

Out of scope

• Don't add silent auto-install of the claude CLI on either platform
• Don't add a Chocolatey or winget package in this PR — those are separate work

Effort: ~3-4 hours.