Add YubiKey Code extension

[nikolas-sturm]

Summary

• Add a native Vicinae extension for YubiKey OATH OTP codes
• Supports copy, paste, and reveal actions
• Uses an external ykoath CLI fast path with ykman fallback
• Provides clear errors when required CLI tools are missing

Validation

• npm run typecheck
• npx vici lint
• npm run verify

Notes

• The extension does not download or bundle binaries. Users install ykoath and/or ykman separately, or configure executable paths in preferences.