Support showing a markdown plan with comment features for Claude harness

I use Claude via the env var `CLAUDE_CODE_OAUTH_TOKEN` from the VS Code chat window. It does work and it does create a plan, but it never shows that plan in the nice markdown view where I can annotate the plan, instead in chat it outputs

```
ExitPlanMode and Write are both unavailable in this session, so I can't write the plan file or formally exit plan mode — here's the plan for your approval instead.
```

And then shows me the plan in chat, where I can't easily annotate it.

The feature request is to fix this, so that I get the nice plan view that I get if I use my copilot harness .