Add `structkit explain` command to preview structure resolution

[httpdss]

Summary

Add a structkit explain command that explains how a structure definition will resolve before generation.

Motivation

generate --dry-run previews file operations, but users also need a structure-focused explanation that shows nested structures, remote content, variables, hooks, and conflict behavior before writing anything.

Proposed behavior

• Accept built-in, custom-path, and local YAML structure definitions.
• Show files and folders that would be generated.
• Show nested structures referenced by folders[].struct.
• Show remote files that would be fetched.
• Show declared variables and resolved values when --vars is provided.
• Show pre/post hooks without executing them.
• Support output formats such as text and JSON.

Example usage

• structkit explain terraform/modules/generic
• structkit explain ./my-struct.yaml --vars project_name=demo
• structkit explain project/python --json

Acceptance criteria

• Command is registered in the CLI.
• Does not create files, folders, or execute hooks.
• Includes tests for nested structures, remote file references, hooks, variables, and JSON output.
• Documentation explains the difference between explain and generate --dry-run.