Primer branding: remove purple glow effect on landing page

[github-actions]

What changed

Color / visual effects

• Removed the .landing-glow decorative background on the landing page (src/index.html, src/styles/style.css). It was a filter: blur(28px) animated radial-gradient combining --agent-purple-rgb, --accent-blue-rgb, and --accent-green-rgb behind the hero heading.
• Removed the @property --thermo-warm-x/-y/--thermo-cool-x/-y custom properties, the thermoDrift keyframes, and the prefers-reduced-motion override that existed solely to animate this glow.

Brand guidance cited

From the primer-brand MCP server's primer_brand_page_design tool (visual-verification checklist, bundled snapshot of @primer/react-brand@0.76.0):

Copy is real and specific, with no glows, purple gradients, pill buttons, or glassmorphism.

The landing glow was exactly this: a blurred, purple-tinted gradient used as ambient decoration behind hero text — an off-brand pattern the guidance explicitly calls out to avoid.

Deviations found but not fixed (follow-ups)

• Pill-shaped elements: .landing-diagram and .ld-phase-icon-multi use border-radius: 999px (pill shapes). The same guidance discourages "pill buttons," but these are non-interactive diagram chips, not buttons, and reworking them risked a larger visual/layout change beyond a focused pass. Left as a follow-up for a dedicated review.
• Agent-purple accent color (--agent-purple, used for the AI-assistant button and copy-success modal) is a Primer product/system color (--color-done-emphasis), not a raw hex value, and is used sparingly (not as a gradient or glow) to signal AI-assisted actions — judged on-brand as-is.
• Typography, spacing, and voice/tone: the primer-brand docs/tokens endpoints in this environment only exposed component-level guidance (Button, page-design checklist, color/type-scale token values) rather than full prose guideline pages (several doc paths returned "could not read"). No hard-coded pixel spacing or off-scale type deviations were found that could be traced to a specific retrieved guideline within that constraint, so no changes were made in those categories this pass.

Validation

• npm test: 326/327 pass. The 1 failing test (copilot-instructions.test.js) fails identically on main before this change (verified via git stash) — a stale generated_at date mismatch unrelated to this PR.
• npm run build: the prebuild vendor-asset fetch (scripts/fetch-vendor-assets.mjs) fails only because this sandbox has no network access to the npm registry — a pre-existing environment limitation. Running npx vite build directly (skipping only the network-dependent vendor step) succeeds cleanly and confirms the HTML/CSS changes compile and render correctly.

Warning

Firewall blocked 1 domain

The following domain was blocked by the firewall during workflow execution:

• registry.npmjs.org

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "registry.npmjs.org"

See Network Configuration for more information.

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