[Feature]: Start workspace from Github Issue

### Problem or use case

Currently, starting a workspace from an issue automatically uses the default agent. Often, depending on the issue’s complexity, type, or my remaining usage limits, I want to start it with a specific agent instead.

### Proposed solution

Add a hover menu to the Start Workspace button that shows the user’s configured agents, allowing them to launch the workspace directly with their chosen agent. Keep the existing default click behavior unchanged.

### Alternatives or additional context

_No response_