Add `structkit lint` command for structure quality checks

[httpdss]

Summary

Add a structkit lint command that performs stricter quality checks on StructKit YAML files than the existing validate command.

Motivation

validate checks whether a structure is syntactically usable. A separate lint command can help maintain higher-quality built-in and custom structures by catching risky, ambiguous, or inconsistent definitions before they are used.

Proposed behavior

• Accept one or more YAML files or structure names.
• Support linting all bundled contrib structures.
• Report warnings and errors with file paths and useful context.
• Support a machine-readable output format, e.g. --json.
• Exit non-zero when lint errors are found.

Initial lint checks

• Missing top-level description.
• Variables referenced in templates but not declared.
• Declared variables that are never referenced.
• Duplicate file or folder entries.
• Unsafe or suspicious hooks.
• Remote URLs that are not pinned to stable refs where applicable.
• Invalid or inconsistent naming conventions.

Example usage

• structkit lint .struct.yaml
• structkit lint structkit/contribs/project/python.yaml
• structkit lint --all
• structkit lint .struct.yaml --json

Acceptance criteria

• Command is registered in the CLI.
• Lint rules are tested individually.
• Supports warning vs error severity.
• Documentation describes lint rules and exit-code behavior.