Fork Conversation opens empty session (resume: undefined) in VS Code extension v2.1.157

Environment

• Extension version: 2.1.157 (darwin-arm64)
• VS Code: latest
• Platform: macOS (Darwin 25.5.0, Apple Silicon)

Bug

"Fork Conversation from here" opens a new tab but the conversation is empty — no history from the fork point is carried over. The new session starts fresh.

Reproduction

1. Have any conversation with at least a few messages
2. Hover over a message and click "Fork Conversation from here"
3. New tab opens but shows an empty conversation (no forked history)

Reproducible 100% of the time across different conversations and different fork points.

Root Cause (from extension logs)

The fork correctly:

• Hard-links file history/checkpoints from the source session to the new session
• Opens a new tab via new_conversation_tab with the correct sessionId

But the spawned Claude process starts with resume: undefined:

Spawning Claude with SDK query function - cwd: ..., resume: undefined

The resumeSessionAt message ID from the fork_conversation request is not being passed through to the new process.

Expected

The forked session should resume from the specified resumeSessionAt message, preserving conversation history up to that point.