Add `structkit vars` command to inspect structure variables

[httpdss]

Summary

Add a structkit vars command that prints variables declared by a structure definition without generating files.

Motivation

Users need to know required inputs before running generate, especially for CI and non-interactive workflows.

Proposed behavior

• Accept built-in, custom-path, and local YAML structure definitions using the same resolution behavior as generate.
• Display variable name, type, default value, description/help text, and whether the value is required or optional.
• Support the existing --structures-path option.
• Support machine-readable output, e.g. --json.
• Exit non-zero when the structure cannot be found or the config is invalid.

Example usage

• structkit vars project/python
• structkit vars ./my-struct.yaml --json

Acceptance criteria

• Command is registered in the CLI.
• Handles structures with variables, structures without variables, custom structures, and local YAML files.
• Includes tests for text output and JSON output.
• Documentation includes usage examples.