Focus Agent Arena dashboard on workshop content

[sishuo-yang]

Summary

• remove the demo-only Countdown and Architecture tabs and their assets/dependencies
• remove the Leaderboard run builder and guided walkthrough, including write APIs and preset config
• add explicit loading states for the run selector and selected-run dashboard data
• update the public workshop documentation and add regression coverage
• ignore local run/review state now that this directory is the single source of truth

Verification

• cd workshops/agent_arena/web && npm test (2 passed)
• cd workshops/agent_arena/web && npm run build
• cd workshops/agent_arena && pytest -q (81 passed)