fix(server): keep linked threads out of the branch pull request lookup

Threads with only already-merged links went to the branch lookup, which
reads the legacy linkedPullRequest reference first. When that reference
was one of the excluded links, its merged summary still settled the
thread. Linked threads skip the lookup again, as before.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
65cafe587aZeus-Deus committed on 10/11/2026, 2:45:14 PMparent7fff5d5

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