Primer branding: fix Analyze button hover contrast failure

[github-actions]

What changed

Accessibility

• .btn-assistant:hover:not(:disabled) (the "Analyze" button in the intent step) swapped its background to var(--agent-purple). In dark mode this token resolves to a light lavender (#c3b5e4), and against the button's white text that produced roughly 1.9:1 contrast — well under the WCAG AA minimum of 4.5:1 for text.
• Fixed by having hover brighten the existing --agent-purple-dark fill (filter: brightness(1.12)) instead of switching color tokens. Contrast is now ~7.3:1 in both light and dark themes.
• Also switched the base .btn-assistant text color from a hardcoded #ffffff to var(--control-fg-on-emphasis, #ffffff) for consistency with the rest of the button system (same visual result, token-based).

Guidance that motivated this change

Retrieved from the primer-brand MCP server:

• primer_brand_tokens returned the base purple scale (--base-color-scale-purple-0 through -9) confirming --agent-purple (#c3b5e4 in dark mode) sits near the light end of the scale — intended for text/accent use, not as a button fill paired with white text.
• Computed WCAG 2.1 contrast ratios directly against these token values to confirm the failure (1.9:1) and validate the fix (7.3:1 light mode base, 10.4:1 dark mode hover).

What I audited but did not change

• All other custom color tokens (--accent-blue, --label-*, --agent-purple-dark) pass WCAG AA contrast in both themes (checked against Primer base-color-scale values).
• The Mona Sans font-family already matches Primer's --brand-fontStack-sansSerif stack.
• The radial-gradient(..., #000, transparent ...) in .is-landing .landing::before is a mask-image alpha gradient (not a visible color), so it's not a branding deviation.
• The purple/blue/green accent palette is internally consistent (all derived from a single aligned hue family) and Primer CSS is loaded from the vendored CDN copy as required — left untouched per the "no new runtime dependencies / no CDN swap" constraint.

Validation

• npm test — 327/327 tests passing (updated one test assertion to match the new hover behavior).
• npm run build (via npx vite build, since the vendor-asset prefetch step requires npm registry access unavailable in this sandbox) — build completes successfully, producing dist/ output with no errors.

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