fix(web-ui): reduce subagent card icon size

[bobleer]

Summary

Reduce subagent card icons from 28px to 20px. The shared agent-control card slot and robot avatars in both Task and agent-control tool cards now use the smaller size.

Type and Areas

Type: UI/UX bug fix

Areas: Web UI, shared design system

Motivation / Impact

Subagent icons were oversized beside their labels. Both the initial placeholder and the robot avatar shown after execution starts now fit the compact card layout, across subagent types.

Verification

All passed:

• pnpm --dir src/web-ui run icons:check
• pnpm --dir src/web-ui run test:run src/infrastructure/design-system/IconUsageIntegration.test.tsx src/app/startup/startupPreload.test.ts src/flow_chat/tool-cards/AgentControlToolCard.test.tsx — 21 tests
• pnpm --dir design-system --filter @openbitfun/ui run test — build and 283 tests
• pnpm run check:web
• pnpm run theme:color-audit:all
• git diff --check

Reviewer Notes

Presentation-only change; no persisted data or protocol changes. Visual inspection and remote workspace, remote control, Peer Device Mode, and Detached Dispatch scenarios were not exercised. Local checks do not establish remote behavior or final visual fidelity.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable (not applicable).