fix: raise Agent mark contrast on the environment overview

[yujiezhang-ops]

Summary

The environment overview drew each Agent mark as a blue glyph on a blue tinted chip. --agent-accent is a single var(--blue) for every row (app.css:1057) rather than a per-Agent brand colour, so the tint carried no information and put the glyph at 3.75:1 on light and 3.82:1 on dark — above the 3:1 graphics threshold, with no margin.

The mark now sits on a transparent box and carries the contrast itself: full black on light, full white on dark, 19.63:1 and 13.94:1 against the card.

--icon-fg would not have worked here. Its #3a3a3c / #d1d1d6 were mixed to sit inside a tinted chip; with nothing behind the glyph the mark needs the full range, so this adds --agent-mark-fg.

Scope is the two overview rules only. The shared .agent-icon base rule keeps its chip, which is what the onboarding Agent list uses.

Notes for the reviewer

The borders are set to transparent, not removed. .agent-icon sizes its 34px box as border-box, so dropping the border hands those 2px to the glyph and silently enlarges the mark.

--agent-mark-fg is added in three places. tokens.css declares the dark palette twice — once for @media (prefers-color-scheme: dark), once for .theme-dark — and a token missing from one copy falls back to the light value when a theme is forced.

No compliance question. All five licensed assets are single-colour glyphs painted with fill="currentColor" and carry no brand colour, so inheriting the page colour is their published behaviour rather than a restyling of vendor artwork. The viewBox assertion in icons/agents.test.tsx still guards the published geometry.

Tests

frontend/src/styles/agent-mark.test.ts locks the three load-bearing facts, none of which had a gate:

1. both card types keep a transparent box at 34px
2. the token resolves black on light and white on dark, in both dark declarations
3. the shared .agent-icon base rule keeps its chip

The assertions read the stylesheets as text: jsdom does not resolve var(), so getComputedStyle cannot see any of this. Each was mutation-tested by reinjecting the prior state — reverting .desktop-app-icon to the tinted chip, deleting the .theme-dark copy of the token, and flattening the base rule instead — and each fails on the matching regression.

Verification

• go vet ./... clean
• 24 frontend test files / 162 tests pass
• tsc --noEmit clean
• vite build clean, one asset pair
• Measured in the browser against the real stylesheets in both themes: box background and border resolve to rgba(0, 0, 0, 0), glyph to rgb(0, 0, 0) / rgb(255, 255, 255), box stays 34×34. The onboarding .agent-icon still measures 38×38 with its border and --icon-fg.

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