feat(studio): visualize sub-agent handoffs

[evanlowe]

Summary

• preserve the emitting Agent across stored and streaming ADK events
• group sub-Agent thinking, tool calls, and responses in a labeled glass-style execution card
• recognize transfer_to_agent as control-flow metadata without rendering duplicate transfer rows
• ship updated Studio assets and presentation contract tests

Testing

• cd frontend && npm test (192 passed)
• cd frontend && npm run build