[Feature] Add a native PowerShell installer for Windows

[Villoh]

Summary

Prime Agent currently documents install.sh, which requires a POSIX shell and leaves Windows users without an equivalent native install path. PR #716 proposed a PowerShell installer that Windows users could run with:

irm https://app.primeintellect.ai/prime-agent/install.ps1 | iex

Proposed behavior

• Publish stable and beta PowerShell installers alongside the existing shell installers.
• Resolve stable, beta, and explicit package versions through the existing release flow.
• Verify the downloaded package tarball with SHA-256 before installing it globally with npm.
• Detect Node.js/npm and offer Node.js LTS or Git for Windows installation through winget when absent.
• Document the Windows command and any Bash-dependent features separately.

Prior work

PR #716 contains an implementation and focused installer validation, including PowerShell 5.1/7 compatibility, release-channel resolution, checksum verification, and fallback plain output. It was closed under the unsolicited-PR backlog policy rather than after a technical review.

Would maintainers want to pursue a supported native PowerShell installer for Windows? If so, I can rework the prior implementation to the accepted scope after an issue and explicit invitation.