[BUG] VSCode: sessions started with slash command never appear in Past Conversations history

Preflight Checklist

• [x] I have searched existing issues 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?

When starting a new conversation in the VSCode extension with a slash command (e.g., /commit, /feature-dev) as the very first message, the session never appears in "Past Conversations" after closing. The conversation works fine in-memory — the skill loads, I interact with it, work gets done — but once I close the tab, it's gone.

Inspecting the .jsonl session file on disk shows only file-history-snapshot entries. No user messages, no assistant messages, no tool calls — zero conversation content persisted. Normal conversations (first message is plain text) always persist and show up fine.

Possibly related to #29331, #27422, #24172, #34163.

What Should Happen?

Sessions started with a slash command should persist and appear in Past Conversations like any other session.

Steps to Reproduce

1. Open Claude Code in VSCode extension
2. Start a new conversation
3. Type a slash command as the first message (e.g., /commit)
4. Interact with the skill — send follow-up messages
5. Close the session/tab
6. Open "Past Conversations" — the session is missing

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.52+

Platform

macOS

Operating System

macOS (Darwin 24.1.0)

Terminal/Shell

VSCode integrated terminal (zsh)