Fix cramped connection-token settings layout

[Yuqing (mydmdm)]

Summary

• add an explicit stacked layout variant to the shared Settings row primitive
• place the Agentlet connection-token description above a responsive wrapping control row
• preserve token actions and cover the responsive DOM contract with focused tests
• document the shared Settings layout contract

Validation

• pnpm --dir apps/web exec vitest run src/components/Settings/sections/ExternalAgentRuntimeSettings.test.tsx (5 passed)
• Web test suite (274 files, 3039 tests passed)
• pnpm typecheck
• pnpm format
• pnpm lint:fix (completed with existing warnings)

Fixes #276