Singleton Config class that manages credential directories, user config (qiita.config.json), root directories, and cache directories with XDG support and environment variable overrides. Includes a Credential class that reads/writes a JSON file with profile-based token management, supporting `QIITA_TOKEN` env var fallback and `0o600` file permissions. Reusable for any CLI tool needing secure config and credential storage.

FileSystemRepo class that manages a local directory of markdown articles, using a hidden `.remote` subdirectory as a mirror of the last known state from the API. Provides load/create/save/publish operations with diff detection against the mirror (via FileContent.equals()), staleness checks, and automatic UUID injection after publishing. The pattern of maintaining a local mirror to detect modifications and prevent overwriting remote changes is a clean, extractable design for any local-remote content sync system.

Two-stage validation for publishing: first checks frontmatter types (via checkFrontmatterType), then validates business rules (title, body, tags, organization+privacy conflicts, campaign agreements), and finally warns if the local file is older than the remote. The layered approach ensures type errors are reported before value errors, making debugging easier. The Validator interface (getMessage, isValid) is a reusable composable pattern for any multi-field validation.

Minimal file-based cache for storing the latest npm package version with a 12-hour TTL. Uses JSON serialization and respects XDG cache directory. Could be extracted and reused by any CLI tool that needs to check for updates without hitting the registry on every invocation.

Renders Marp-generated slide HTML inside a Shadow DOM to prevent app styles from leaking into or being overridden by the slide content. Uses React portal to inject the style tag and slide HTML into the shadow root. This is a non-obvious solution to a common problem when embedding third-party HTML with global CSS (like Marp) into a React application.