Build a Small CLI Tool With Tests

Problem Build a small command-line tool (e.g. a config linter or a simple kv client) with clean structure and unit tests, then extend it with an additional subcommand.

Input / Output

• Input: command-line arguments and a config/input file or key-value operations.
• Output: parsed/validated results, clear error reporting, and a working new subcommand.

Constraints

• Idiomatic Go; the code should be modular, testable, and extensible.
• Emphasis on real-world engineering (packages, error handling, tests) rather than algorithmic tricks.

Example

• parse args -> validate config -> report errors, then add a new subcommand cleanly.