feat: simplify VS Code eval runner

[christso]

we should require users to run VS Code evals using a non-VS Code agent such as Codex or Copilot CLI.
Instead of provisioning subagent workspaces, simply open the original workspace file. This significantly reduces indirections.

[christso]

actually maybe we should deprecate VS Code runner. Too much trouble than it's worth. VS Code users can use the agent mode orchestrator.