fix(app): animate subagent card chevron

[iamdavidhill]

Summary

• replace the subagent card external-window icon with a right chevron
• reserve the chevron space inside the card to prevent layout shifts
• fade and slide the chevron by 4px on hover with RTL and reduced-motion handling

Testing

• bun typecheck (packages/app)
• bun test src --only-failures (packages/session-ui): 177 passed
• production timeline benchmark before and after: 4 passed

Note

• packages/session-ui typecheck is blocked by existing unresolved vite-plugin-solid imports in performance fixtures.