fix(server): a portable fork with no surviving history rejects /compact

Counting every run fork as conversation let a portable fork whose handoff
budget omitted all inherited messages compact an empty thread. A fork now
counts only when it resolves natively or its portable handoff carries at
least one message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2615e21decJulius Marminge committed on 10/23/2026, 4:59:38 PMparent4bc67e6

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