Fixes#25950.
## Why
Forking a renamed thread could fall back to the source thread's
first-prompt title because the fork path did not preserve the source's
explicit name. That meant fork-of-renamed-fork flows could show stale
sidebar labels even though the user had renamed the parent.
## What changed
`thread/fork` now reads the source thread's distinct `name`, normalizes
it, persists it onto materialized forks, and applies it to the returned
API thread. Because the source `name` already excludes first-prompt
pseudo-titles, forks inherit only an explicit user rename instead of
stale generated metadata.
Eric TrautandGitHub
d8121f93c8