Use unique names for workspaces by default

Right now, the default workspace name is `issue-${issue_number}`. This prevents users from starting two workspaces in parallel for the same issue, since each workspace must have a unique name.

We can address this by appending numbers to new workspaces with clashing names, like `issue-${issue_number}-2`. In contrast to appending a random suffix to each workspace name, this solution keeps the names clean and human-readable.