Add `s-divider` component

[auswm85]

Problem

Users need a simple way to visually separate content sections without writing custom CSS for horizontal or vertical dividers.

Proposed Solution

Add an s-divider component for horizontal and vertical separators:

<!-- Horizontal (default) -->
<hr s-divider>

<!-- Vertical (for flex rows) -->
<div s-divider="vertical"></div>

<!-- With spacing variants -->
<hr s-divider class="s-my-4">
<hr s-divider class="s-my-8">

Features:

• Default horizontal line using border-top or pseudo-element
• Vertical variant for use in flex containers
• Uses --s-neutral-* tokens for color
• Respects light/dark mode via light-dark()
• Minimal height/width (1px default)

Alternatives Considered

• Using <hr> with custom CSS (requires per-project styling)
• Border utilities (less semantic)

Additional Context

Keep it simple - just a styled line. No text labels or fancy variants needed for v1.

Suggested location: packages/core/src/components/divider.css