Add 'vars' command to inspect structure variables

[httpdss]

Motivation

• Provide a way to inspect the input variables declared by a structure definition before running generate so users and CI can see required inputs without creating files.
• Support the same resolution rules as generate (built-in contribs, custom --structures-path, and local YAML files) and offer machine-readable output for automation.

Description

• Add a new VarsCommand in structkit/commands/vars.py that loads a structure YAML, normalizes variables, and prints either aligned text or JSON via --json.
• Implement robust structure resolution and error handling for missing files, invalid YAML, and malformed variables entries, exiting non-zero on failure.
• Register the vars subcommand in structkit/main.py and add tests in tests/test_commands.py covering text output, JSON output, no-variables, custom --structures-path, and invalid-config failure.
• Document the command and examples in docs/cli-reference.md and docs/usage.md.

Testing

• Ran the full test suite with pytest -q and observed all tests passing (159 passed).
• Ran the focused command tests with pytest tests/test_commands.py -q and observed them passing (20 passed).
• Verified the CLI invocation python -m structkit.main vars structkit/contribs/github/workflows/run-struct.yaml --json returns the expected JSON output.

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Codex Task

[httpdss]

@codex, please review

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[httpdss]

@codex, please review

[chatgpt-codex-connector]

To use Codex here, create a Codex account and connect to github.