New Markdown UI element with clickable links for plugin/skill interfaces

What

• A new Markdown element type is available in the UI toolkit plugins and skills use to build custom terminal interfaces, alongside existing elements like Box, Text, and Code. It supports props such as text, dimColor, and pressableLinks, and its content is sanitized to reject control characters.
• Markdown text can now contain clickable (pressable) links: a new renderer recognizes https://, http://, and file:// links, distinguishes single- from double-clicks, and calls a press handler with the link's destination when clicked.
• The system that finds which UI elements are clickable now also collects a pressableLinks list of link destinations for a pressable element.
• The Markdown component is now an allowed element on the default, mobile, and VS Code render surfaces, with a dedicated error if a Markdown element with links is missing its click handler.

Why This lets plugins and skills present richer, formatted text with clickable links in their custom terminal UI, instead of being limited to plain text or buttons.