feat: callout/admonition block type

[oobagi]

Problem

The editor has no native way to create callout/admonition blocks (tip, warning, note, info). GitHub-style admonitions are rendered in view mode (#21), but there's no dedicated block type — users can't create them from the command palette or edit them structurally.

Goal

Add a Callout block type with variants (note, tip, warning, caution, important) that renders as a colored box with an icon and label.

Markdown format

GitHub-style blockquote admonitions:

> [!NOTE]
> This is a note callout.

> [!TIP]
> Helpful advice here.

> [!WARNING]
> Watch out for this.

Tasks

• Add Callout to BlockType enum with variant tracking (note/tip/warning/caution/important)
• Parse > [!TYPE] syntax in parse.go — detect admonition prefix, store variant + content
• Serialize callouts back to GitHub blockquote admonition syntax
• Round-trip tests
• Add Callout entry to / command palette (with variant sub-menu or default to Note)
• Render inactive callout with colored left border, icon, and label
• Add CalloutStyle to theme system (per-variant colors)
• View mode rendering with full styling

Scope

Type: enhancement
Size: medium