/branch command replaces original conversation instead of keeping both

Description

When using the /branch command on an existing conversation, the original conversation disappears from the session resume list and only the new branch appears. The expected behavior is that both the original conversation and the new branch should be listed.

Steps to Reproduce

1. Have an existing conversation (e.g., named "main blog s3 agent")
2. Run /branch within that conversation
3. A new branch is created (e.g., "main blog s3 agent (git pr review) (Branch)")
4. Go back to the session resume list

Expected Behavior

Both conversations should appear in the resume list:

• "main blog s3 agent" (original)
• "main blog s3 agent (git pr review) (Branch)" (the new branch)

Actual Behavior

Only the branch appears:

• "main blog s3 agent (git pr review) (Branch)"

The original "main blog s3 agent" conversation is no longer listed and cannot be resumed.

Environment

• OS: macOS (Darwin 25.3.0)