feat: link bookmark block type

[oobagi]

Problem

Pasting a URL into the editor creates a plain text block. There's no way to create a styled link card that shows the URL prominently.

Goal

Add a Bookmark block type that renders a URL as a styled card with the link title and URL displayed.

Markdown format

Store as a standalone URL line or a titled link:

[Bookmark Title](https://example.com)

Tasks

• Add Bookmark to BlockType enum
• Parse standalone URLs and titled link syntax into Bookmark blocks
• Serialize back to markdown link syntax
• Round-trip tests
• Add to / command palette with URL input prompt
• Render as a bordered card showing title + URL (similar to code block box style)
• Add BookmarkStyle to theme system (border, title color, URL color)
• View mode rendering

Design considerations

• Phase 1: manual title entry — user types the title and URL
• Phase 2: auto-fetch page title from URL on paste (optional, requires network)
• OSC 8 clickable links in terminals that support it

Scope

Type: enhancement
Size: medium