fix: install.ps1 compatibility and improved path resolution

[anirudhpv]

This PR addresses two issues in the Windows installation script to improve the user experience for standard Windows environments.

Changes:

1. PowerShell 5.1 Compatibility: Replaced -LiteralPath with -Path in the New-Item call. The -LiteralPath parameter for New-Item was introduced in PowerShell Core (7.0+) and causes failures on the default Windows PowerShell 5.1.
2. Flexible Path Resolution: Updated the root directory detection to check for GEMINI.md in the current folder before falling back to git rev-parse. This allows the assistant to be installed correctly when it is nested within a larger project subdirectory.

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