Feature: tool-level documentation and examples link in generated GUIs #62

Goal

Let a scientific tool declaration link to an official documentation page or example without embedding host-specific markup in its @tool function signature.

Design as optional metadata on the tool (not each parameter by default), with a help_url or equivalent field exposed by labconstrictor-tools check, the manifest and host GUIs. A label such as Documentation or Examples may open the URL through the host's normal browser integration. Reuse docstrings and Description for short in-form help.

Acceptance criteria

• Schema author can provide a safe, validated HTTPS documentation URL; optional package-relative documentation only if there is a robust cross-host resolution policy.
• Fiji, Napari and QuPath expose an unobtrusive and consistently named Help link; CLI and notebook display or report it appropriately.
• A missing URL changes nothing; bad URLs fail declaration validation, not at click time.
• The URL is metadata only: never fetched or executed by the worker, and applications must not need a per-host plugin.

Priority: Medium/low; small additive feature.