[Bug] Agent initialization overwrites custom session title

Title: statusline-setup agent overwrites session title

Description:

The statusline-setup agent runs automatically at session start and sets the session's custom-title to
"statusline". This overwrites the real title that should come from the user's first message or task. In
long-running sessions where /clear is used and the agent re-triggers, it can also overwrite a meaningful
title that was already set.

Reproduction:

1. Open a session in any project that has a statusline configured
2. Check the session JSONL — the first custom-title entry is "statusline" before any user interaction
3. If the session is short or the auto-titler doesn't fire, the session list shows "statusline" as the

name

Impact:

Multiple sessions in the history all named "statusline", making the session list hard to navigate. In my
case, 3 out of 10 recent sessions had this problem.

Expected behavior:

Background/utility agents like statusline-setup should not set the session title. Only the auto-titler
(from user messages) or explicit user renames should control it.