fix(cli): parse color-mix output in contrast audit

[miguel-heygen]

Summary

• Parse Chromium modern color(srgb ... / alpha) computed-color output in the contrast audit.
• Preserve support for comma- and space-separated rgb/rgba values and percentages.
• Keep the standalone creative-skill contrast report in sync, including the documented decorative-text opt-out.
• Refresh the skills manifest.

Root cause

Current Chrome preserves color-mix(in srgb, X, transparent) as color(srgb r g b / a). The audit parser recognized only rgb()/rgba(); every other computed color silently became opaque black, so audit diagnostics disagreed with the rendered pixels.

Tests

• Browser reproduction confirms Chrome returns color(srgb 1 0 0 / 0.5).
• Exact red/green audit regression: prior foreground rgb(0,0,0), fixed foreground rgb(255,128,128) over white.
• Contrast/layout browser suite: 65 passed.
• Full CLI suite: 1,746 passed, 2 skipped.
• CLI typecheck, formatting, lint, diff check, and pre-commit checks passed.

Human review is required; this PR is not self-approved or merged.