/clear and "Clear conversation" create a new empty session instead of clearing the current one

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Environment:
Claude Code version: 2.1.246 (extension cache also shows 2.1.273 present)
Platform: macOS (darwin-arm64)
Interface: VS Code extension, Session Manager sidebar UI

Expected behavior:
The current session's conversation history is cleared/truncated in place, keeping the same session active with an empty context (so token/context usage actually resets).

### What Should Happen?

Actual behavior:
A brand new session is created (new session ID, often inheriting the old session's custom title), while the original session remains fully intact and un-cleared underneath. The new session's .jsonl transcript contains only boilerplate:

### Error Messages/Logs

```shell

```

### Steps to Reproduce

Steps to reproduce:

Open an existing session with conversation history in the VS Code Claude Code panel
Either type /clear and submit, or select "Clear conversation" from the / command autocomplete panel
Observe the sidebar's session list

### Claude Model

Sonnet (default)

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.246

### Platform

Other

### Operating System

macOS

### Terminal/Shell

VS Code integrated terminal

### Additional Information

_No response_

Start by reproducing the behavior in the VS Code Claude Code panel, using both the /clear command and the "Clear conversation" entry in command autocomplete. Trace the Session Manager's clear-conversation entry point and inspect the resulting session ID and .jsonl transcript; done means the existing session is emptied in place, its context usage resets, and no new session is created.

Written by the indexing model from the issue text.