🤖 fix: best-of grouped finalization can ship a report artifact a fast sibling continuation already replaced

[ibetitsmike]

Summary

Follow-up from Codex on #4058 (round 23, P1), filed rather than fixed there under Mike's stopping rule for that PR (best-of orchestration is outside that PR's startup/shutdown thesis).

TaskService.buildBestOfCompletedTaskToolOutput (src/node/services/taskService.ts around line 12813 on b795316c1e) reads every sibling's report artifact, then re-checks that no non-reporting sibling is executing again (isBestOfSiblingExecutingAgain) before assembling the grouped task-tool output. Codex:

When a sibling continuation starts after its old artifact was read and finishes quickly enough to write a replacement before this final pass, the sibling is no longer executing or streaming, so this check succeeds and the parent is permanently finalized with the old report already stored in reports. [...] the second pass samples only current activity and never verifies that each artifact is still the revision that was read; compare/re-read artifact identities or serialize assembly with sibling continuation completion.

Suggested direction

Either re-read each artifact after the activity check and compare a revision marker (nowMs/persistedAtMs from upsertSubagentReportArtifact) with the one first read, or take the per-child report lock (or deferredBestOfLocks) around the whole assembly so a continuation's report write cannot interleave with it.

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[linear-code]

MUX-129