Custom chat names should not be auto-overwritten

Problem

After renaming a chat to a custom name, subsequent model-generated titles overwrite the user's chosen name.

Why this matters

Custom names exist specifically so users can remember what a chat is about. Auto-overwriting them defeats the entire purpose of the feature — if the name can change without user action, it stops being a reliable identifier.

Example: I had a chat renamed "Anamnesis-δ²" for ongoing work on a specific neural engine project. After a later session the name was replaced automatically, making the chat much harder to locate among many others.

Expected behavior

Once a chat is renamed by the user, only the user should be able to rename it. Auto-generated titles should only apply to chats the user has not named.

Actual behavior

Custom-named chats can be renamed programmatically, losing the user's chosen label with no undo path.