Add structkit lint command and MCP tool

[httpdss]

Motivation

• Provide a stricter static quality/safety checker for StructKit YAML definitions to catch risky patterns that validate does not (duplicate entries, undefined template variables, unsafe hooks, unpinned remote URLs, etc.).
• Expose lint functionality over MCP so external tools and automation (including AI workflows) can run lint checks programmatically.

Description

• Add a new CLI command implemented in structkit/commands/lint.py (LintCommand) that lints YAML files or structure names with options --all and --json, and exits non-zero on lint errors.
• Register the new lint subcommand in the main parser via structkit/main.py and update docs in docs/cli-reference.md and docs/mcp-integration.md.
• Add an MCP tool lint_structure in structkit/mcp_server.py with transport-agnostic logic _lint_structure_logic and a compatibility handler _handle_lint_structure that return results in text or JSON.
• Lint rules include: missing top-level description, undefined/unused variables, duplicate files/folders/variables, template syntax errors, invalid YAML, invalid hook shapes, suspicious/unsafe hooks (e.g., piping remote script to shell, rm /), unpinned GitHub raw/blob references, and variable naming conventions; Jinja filters/globals are stubbed for safe static parsing.

Testing

• Ran the full test suite with pytest and all tests passed (170 passed).
• Added unit tests for the CLI and MCP integration: tests exercising lint JSON output, error exit behavior, CLI parser registration, MCP tool registration, _lint_structure_logic and its compatibility handler; these tests passed under CI runs.
• Manually exercised the command with python -m structkit.main lint structkit/contribs/terraform/modules/generic.yaml --json and python -m structkit.main lint --all --json to validate JSON/text output and summary counts, which produced expected results.

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Codex Task