Primer branding: remove off-brand purple gradients and glassmorphism

[github-actions]

What changed

Color — Removed off-brand purple/indigo gradients and glassmorphism, flagged directly by the Primer Brand review tool (primer_brand_review):

• .btn-assistant (rest, hover, disabled): replaced linear-gradient(135deg, var(--agent-purple), var(--agent-purple-dark)) with a flat var(--agent-purple-dark) fill; hover now lightens to var(--agent-purple) instead of a second gradient/shadow layer.
• .copy-modal-card: replaced a radial purple gradient + box-shadow combo with a flat var(--bg-card) surface and a solid var(--agent-purple) 1px border.
• .copy-modal::backdrop: removed backdrop-filter: blur(4px) (glassmorphism).
• .copy-modal-icon, .copy-modal-action: replaced purple gradients with flat var(--agent-purple-dark) fills.
• .option-card.priority-archetype: replaced a linear-gradient blue wash with a flat rgba(var(--accent-blue-rgb), 0.05) tint (same subtle highlight, no gradient).

All colors already came from existing site accent variables (--agent-purple, --agent-purple-dark, --accent-blue-rgb), so no new hex values were introduced — only the gradient/shadow/blur techniques were removed.

Guidance that motivated each change (from the primer-brand MCP server)

Ran primer_brand_review against the modified CSS sections. It returned these warnings, which drove the fixes:

• off-brand-gradient: "A purple/indigo gradient is the classic off-brand 'SaaS' tell and is penalized hard. Use neutral surfaces with a sparing functional accent." → removed all .btn-assistant / .copy-modal-* purple gradients.
• shadow-and-gradient: "Combining box-shadow with a gradient is a retired GitHub style. Prefer flat surfaces with thin 1px borders." → .copy-modal-card now uses a flat surface with a 1px border instead of a gradient + large box-shadow.
• glassmorphism: "Glassmorphism / frosted blur is off-brand. Use flat surfaces separated by subtle 1px borders." → removed backdrop-filter: blur(4px) on the modal backdrop.

After the fixes, a full-file primer_brand_review re-run confirmed 0 errors and only pre-existing, lower-risk warnings remain (see below).

Deviations found but deliberately not fixed

• hardcoded-hex on #ffffff, #f6f8fa, #24292f, #57606a, etc.: these are all var(--color-*, #hex) fallback values for Primer CSS custom properties (the standard progressive-enhancement pattern used throughout this file for browsers/contexts where @primer/css variables aren't set), not raw hardcoded colors. Leaving as-is to avoid breaking the fallback chain.
• hardcoded-px (various pixel sizes for icon dimensions, border widths): these are fixed-size UI elements (icon squares, focus outlines) rather than layout spacing, and don't have an obvious Primer spacing-token equivalent without a larger refactor. Flagged as a follow-up.
• pill-button (border-radius: 50% on .copy-modal-close): this is a small circular icon-only close button (×), not a CTA/label pill. Left as-is since it doesn't read as an off-brand rounded button in context.
• The landing hero's blue→green gradient text (.landing-title) and ambient thermo glow (.landing-glow) were reviewed separately with primer_brand_review and passed clean (0 warnings for gradient/shadow/glass) — they use only Primer-aligned accent colors (blue, green) in one family, so they were kept per the "tasteful shine" allowance.

Tests / build

• npm test: all 323 tests pass. Two existing tests asserted on the specific linear-gradient/var(--agent-purple) implementation details that this PR intentionally changes; they were updated to assert on the new flat-color implementation while preserving the same behavioral intent (subtle highlight present, hover/disabled colors correct).
• npm run build: the Vite bundling step itself succeeds (verified directly via npx vite build). The prebuild step (scripts/fetch-vendor-assets.mjs) failed in this sandbox because outbound npm pack requests to the npm registry return 403 Forbidden — this is a pre-existing environment/network restriction, reproduced identically on a clean main checkout with no changes applied, and is unrelated to this change set.

Warning

Firewall blocked 1 domain

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• registry.npmjs.org

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

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

See Network Configuration for more information.

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