Add Windows support via chezmoi

- Add .chezmoiroot to preserve stow structure
- Add .chezmoiignore to exclude Linux-only packages on Windows
- Add PowerShell scripts for path conversion and skills symlinks
- Update README with Windows installation instructions
- Tested: vim, agents, codex, claude, opencode configs work on Windows

.gitignore

@@ -1,4 +1,5 @@
 .DS_Store
+nul
 
 # Secrets
 .env.secrets