docs: README overhaul (brand header, dash-free, accuracy fix)

[EthanY33]

Recruiter-facing README pass. The repo already had a strong animated hero (demos/overview.webp), so it was kept; no new header image added.

Accuracy fixes (verified against source):

• accessibility-design-audit: corrected "emits markdown report + screenshots" to "markdown report grouped by impact plus raw axe JSON". The skill (plugins/atelier/skills/accessibility-design-audit/index.mjs) writes a11y-report.md and a11y-raw.json and takes no screenshots.
• Test count: "52 tests (3 skipped if ffmpeg not present)" corrected to "59 tests (5 require ffmpeg, skipped when it is absent)". Ran the suite: 58 passed + 1 skipped (59 collected); the 5 ffmpeg-gated tests live in html-to-video.
• Reworded the hero claim from "hero GIF" to "hero clip" since the embedded asset is a WebP, not a GIF.

Prose cleanup:

• Removed all 6 em dashes from prose, converting to plain punctuation (periods, colon, comma). Kept hyphens in genuine compounds and the → / + glyphs.

Verified: skill count (7) and MIT license match package.json, plugin.json, and LICENSE; all relative links resolve on disk.