feat: add install script for Windows (PowerShell) #70

Problem

No automated way to install the skill on Windows. Users must manually create directories and copy/symlink SKILL.md.

Expected behavior

Add scripts/install.ps1 that:

• Creates ~/.claude/skills/git-workflow/ (resolving `~` to `$env:USERPROFILE`) if it doesn't exist
• Creates a symlink to SKILL.md using `New-Item -ItemType SymbolicLink`
• Detects if already installed and handles gracefully
• Supports `-Uninstall` parameter to remove the symlink
• Prints clear success/failure messages
• Handles the Windows requirement of elevated privileges for symlinks (or falls back to copy with a warning)

Notes

Windows symlinks require either Developer Mode enabled or an elevated terminal. The script should detect this and provide guidance if symlink creation fails.