Limit command

### Feature summary

Add /limit command on Github copilot App

### What problem are you trying to solve?

In the Copilot App the command /limit is missing.
I want to set a maximum AI credits for a session.

### Proposed solution

When I set a limit of credits consumption, knowing that the usage is checked after model calls return, so one call can exceed the limit, I want to see the agent stop a task when the limit is reached, so I can put a safe stop on a long session.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

The /limit command already exists on copilot CLI

Start by finding the existing /limit command in the Copilot CLI and compare its behavior with the Copilot App command handling. Implement the equivalent session credit limit, accounting for usage checked after model calls return, and verify that a task stops once the configured limit is reached.

Written by the indexing model from the issue text.