Components: ship `.sf-skeleton`

[jackgranatowski]

Type: components layer · Tier: v1.0 core

Summary

Ship .sf-skeleton loading-placeholder component.

Why

Pure-CSS shimmer animation — a perfect fit for the no-JS contract and an easy, visible win.

Verified state (2026-06-20)

Reserved commented stub — 0 live rules.

Constraints

Additive, BEM-first, no build, no JS. Modifiers per docs/components.md: --text, --avatar, --card, --line. Must respect prefers-reduced-motion.

Acceptance criteria

• Implement .sf-skeleton + shape modifiers
• Shimmer disabled under prefers-reduced-motion
• Tests (light + dark)
• Documented in docs/components.md
• MINOR bump only