Support multiple clients simultaneously connected to the same session

[justyn]

Could we add the ability to be connected to the same executing agent session with more than one client at the same time?

This would allow for extremely adaptable use of Goose and would be particularly useful for any long-running tasks.

Example use cases

• Start a long-running task in CLI or Desktop then monitor via Web on mobile (eg using Tailscale) while away from PC
• Begin sketching out an idea or triggering a task on Web on mobile then get out laptop to continue via CLI/Desktop
• Start a task in Desktop then jump over to CLI to use native CLI editor
• Start a task in CLI then jump over to the Goose Voice Client (....one day)

Proposal

This could presumably be based on the work being done already as part of #4389 eg AgentManager.

The key would be for the AgentManager to create a multi-consumer broadcast channel for each session's events, and for the agents themselves to publish to this instead of directly to the client.

Potentially, it might also be desirable to track the "active" client, with the others read-only until handoff is requested.