fix(server): a fork's first /compact compacts its inherited history

A forked thread starts with its source's conversation but no messages of
its own, so getTurnStartContext reported hasConversation=false and the
first /compact failed as an empty thread. Count a run fork as having
conversation in both projection stores.

Fixes #24160

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
e92888749dJulius Marminge committed on 10/23/2026, 4:31:10 PMparentcf0b9de

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