Add graph command to visualize structure dependencies

[httpdss]

Motivation

• Provide a way to inspect how folders[].struct references compose structures so users can reason about nested dependencies, detect missing references, and identify cycles.
• Support both human-readable and machine-consumable outputs (text tree, JSON, Mermaid) so the data can be used in docs, CI, and tooling.

Description

• Add a new GraphCommand implemented in structkit/commands/graph.py that resolves structure paths, extracts folders[].struct dependencies, detects cycles, and records missing references.
• Support CLI options --all, --format {text,json,mermaid}, and -s/--structures-path, and hook the command into the main parser via structkit/main.py.
• Implement renderers for text tree, JSON (format_json), and Mermaid (format_mermaid) including Mermaid classes for missing references and cycles, and safe node id/label escaping.
• Add comprehensive tests in tests/test_graph_command.py and update docs (README.md, docs/usage.md, docs/cli-reference.md) with usage examples.

Testing

• Ran pytest tests/test_graph_command.py, which passed (all tests green).
• Ran the full test suite with pytest, which passed (163 passed).
• Exercised the CLI manually with python -m structkit.main graph --help, python -m structkit.main graph terraform/apps/generic --format mermaid, and python -m structkit.main graph --all --format json, all producing the expected outputs.

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Codex Task