feat: Given that there's already a default agent for the city, I shouldn't have to re-specify it when invoking gc commands (e.g., gc sling claude "Do something"

[donbox]

Before you continue

• I searched existing issues and did not find a duplicate.
• I can explain the user problem this solves, not just the implementation I prefer.

Problem to solve

Conceptual overhead plus extra characters to type.

We've already designated a default agent of the city - let's use it.

Proposed change

use the default agent for the city if not specified in the command line for gc

Alternatives considered

No response

Scope and impact

No response