Add terminal demo GIF/video to README

[lewiswigmore]

The README needs a short (30-60 second) terminal recording showing Cicada in action:

1. Running \cicada\ to launch a team
2. The 4-agent pane layout appearing
3. Agents coordinating via MCP (messages, task board)
4. Monitor sidebar showing live activity

Tools: VHS, asciinema, or OBS screen recording.

This is the single biggest driver for GitHub stars — people need to see it in action.