Agent Window: Provide more context in the subagent dropdown

The session has been going on for a while, and it created sub-agents along the way for multiple purposes, including adversarial review for planning, implementation, and other areas. This menu is basically impossible to use at this point because I don't know which sub-agent was just created, which is running, and which recently returned.

Suggestions:
- Running state and/or time-since-last-response: Important to sort to the top for quick access.
- Model: Important for cost awareness and many subagent use cases are model-specific; which in this list shows as duplicated entries where tasks got assigned to 2 models for debate.
- (Bonus: All-up tokens stats, as subagents can generate hidden cost while running in background cc @cwebster-99 )