Allow default agent selection with /agent without interactive list #5141

Describe the feature or problem you'd like to solve

Selecting the default agent using the /agent command directly, without opening the interactive agent list

Proposed solution

While custom agents can be selected by typing /agent agent-name <enter>, the default agent can only be selected by typing /agent <enter> (the interactive agent list pops) <enter>.

In general, this behavior is acceptable one you get used to it, even if represents different treatment for the default agent. However, but this can become an annoyance if one wants to enqueue (<ctrl>+q) the selection of the default agent, because that would enqueue the popping of the agent list, not the default agent itself.

A possible solution could be a syntax like /agent . (dot) or /agent - (dash) or /agent default (if we assume that default is not a custom agent name).

Example prompts or workflows

1. Stuff is running in a custom agent
2. /agent . <ctrl>+q
3. Paint my dragon with rainbow colors using unicorn brush <ctrl>+q
4. After custom agent has finished, default agent is selected and executes queued prompt

Additional context

No response