Add lint command for StructKit YAML definitions

[httpdss]

Motivation

• Provide a targeted linting command to detect quality and safety issues in StructKit YAML definitions that go beyond schema validation, such as unpinned remote URLs, risky hooks, undeclared/unused variables, duplicates and naming problems.

Description

• Add a new CLI subcommand lint implemented in structkit/commands/lint.py with support for multiple targets, --all, -s/--structures-path, and --json output.
• Implement a suite of lint checks including baseline schema validation via ValidateCommand helpers, missing top-level description, duplicate file/folder entries, undeclared and unused template variables, suspicious/unsafe hook patterns, unpinned GitHub/raw URLs, and invalid entry names.
• Register the lint subcommand in get_parser() in structkit/main.py and document the command, rules, usage and examples in docs/cli-reference.md.
• Add focused unit tests in tests/test_lint_command.py that cover command registration, variable/template checks, duplicates, hook and remote URL detection, JSON output and error exit behavior, and --all discovery.

Testing

• Ran pytest tests/test_lint_command.py and the tests passed (7 passed).
• Ran the full test suite with pytest and all tests passed (161 passed).
• Exercised the CLI end-to-end with python -m structkit.main lint --all --json which produced a JSON report of warnings/errors and returned non-zero exit when errors were present as expected.

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Codex Task