add real-time web dashboard with WebSocket live updates

Standalone Bun.serve() on localhost:7900 with 6-tab interface:
Agents, Messages, Plan, Knowledge, Files, Stats. Dark theme,
responsive layout, keyboard shortcuts (1-6 to switch tabs).

- dashboard/server.ts: fetches broker state every 500ms, pushes
  to all connected browsers via WebSocket. Session resolution,
  API endpoints for session switching and guardrail adjustment.
- dashboard/index.html: self-contained HTML with inline CSS/JS,
  zero external dependencies. WebSocket client with exponential
  backoff reconnection and keepalive ping.
- Orchestrator auto-launches web dashboard alongside TUI on
  create_team and resume_session.
- CLI: `multiagents web [session-id]` command.
- 12 new tests covering HTTP serving, API endpoints, WebSocket
  state push, ping/pong, and knowledge data inclusion.