Add `structkit graph` command to visualize structure dependencies

[httpdss]

Summary

Add a structkit graph command that shows dependency relationships between structure definitions.

Motivation

StructKit structures can compose other structures through folders[].struct. As templates grow, users need a way to understand dependencies, detect cycles, and document how structures are composed.

Proposed behavior

• Accept a structure name, local YAML file, or --all.
• Traverse nested structure references from folders[].struct.
• Detect and report missing references and cycles.
• Support output formats such as text tree, JSON, and Mermaid.
• Support custom structures through --structures-path.

Example usage

• structkit graph project/python
• structkit graph terraform/apps/generic --format mermaid
• structkit graph --all --format json

Acceptance criteria

• Command is registered in the CLI.
• Includes tests for single dependencies, multiple dependencies, nested dependencies, missing dependencies, and cycles.
• Mermaid output can be pasted into documentation.
• Documentation includes examples for text, JSON, and Mermaid output.