feat: make the package ready for external use

[vreshch]

Why

@agentage/design-system is published to npm with --access public, but the package carried
"license": "UNLICENSED" and shipped no LICENSE file. Legally that means nobody outside the org
could use it. It also had no keywords (zero npm discoverability), no homepage, no bugs URL,
and a README written for the two monorepos that used to vendor the package rather than for someone
installing it for the first time.

What

Licensing

• LICENSE - MIT, matching @agentage/find-mcp and the rest of the public packages.
• package.json license set to MIT.

npm metadata

• homepage -> https://ds.agentage.io, bugs -> the issues URL.
• repository.url in the conventional git+https:// form.
• 16 keywords covering design-system / react-19 / tailwind-v4 / oklch / rsc / a11y.
• Description now states what the package is rather than repeating the title.
• Key order follows the npm convention (identity -> discovery -> distribution). The generated
  108-entry exports map is untouched.

README - rewritten on the find-mcp model: banner, what-it-is, four-step getting started
(install / theme / the Tailwind @source trap / first component), a component table linking each
of the eight categories to its live page, the RSC contract, theming tiers, a requirements table,
develop + release, contributing, license.

Banner - docs/banner.svg, built from the actual OKLCH gold scale converted to sRGB.

Showcase header - a copyable npm i @agentage/design-system chip (dogfooding CopyButton)
plus npm and GitHub links. The chip is gated at xl so the header stays on one line at 1024px;
nav labels get whitespace-nowrap, which also fixes a pre-existing two-line wrap of
"Getting started" at that width.

Verification

• npm run verify green - type-check (src + dev + e2e), lint (0 errors; the 5 warnings are
  pre-existing react-hooks/set-state-in-effect in src/), format, exports:check, 434 tests,
  build.
• npx playwright test in mcr.microsoft.com/playwright:v1.62.1-noble: 43 passed, including
  the axe WCAG A/AA scans and all 11 screenshot baselines. No baseline regeneration needed - the
  visual snapshots scope to content regions, not the header.
• Header checked in a real browser at 1440 / 1280 / 1024 / 820 and in both themes.
• Copy button verified functionally: clicking it puts npm i @agentage/design-system on the
  clipboard. Both external links resolve to the right URL with target="_blank".
• Banner rasterized and eyeballed; the wordmark glyph spacing was fixed after the first render.

Note

This does not bump the version. The metadata only reaches npm on the next chore(release):
commit - worth doing soon, since every currently published version is still marked UNLICENSED.