feat: Add auto-cache, fix cache restore, refactor help system

- Add AUTO_CACHE=Y/N setting in .env to auto-cache ISOs on stop
- Fix cache restore to copy metadata files (windows.base, windows.ver,
  windows.mode, windows.type, windows.args) alongside ISOs so the
  container recognizes them as already processed and skips re-download
- Refactor show_usage() into topic-based help with interactive menu
  (commands, instances, cache, examples, config) and aligned columns
- Fix clean --data to unregister instances and remove compose files
- Update WINCTL_GUIDE.md and readme.md with all changes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

readme.md

@@ -134,6 +134,7 @@ Use `winctl.sh` for easy container management:
 ./winctl.sh cache list                 # Show cached ISOs
 ./winctl.sh cache rm winxp             # Remove cached winxp ISO
 ./winctl.sh cache flush                # Clear all cached ISOs
+# Or set AUTO_CACHE=Y in .env to cache ISOs automatically on stop
 
 # Full help (includes ARM64 info)
 ./winctl.sh help