[codex] Split turn rollups at user boundaries

[brsbl]

Summary

• Track human user messages from other turns as hard boundaries for completed turn summaries.
• Split completed Worked for... summary segments at those boundaries so background parent turns cannot wrap output across a later user message.
• Keep enough summary detail only for crossed turns, then sort projected rows by source sequence after segmentation.
• Add a regression that reproduces the observed shape: $ottonomous:review parent turn, overlapping background worker turn, later user new-turn, then more parent output.

Root Cause

Completed turn summaries were projected as one monolithic turn row anchored at the parent turn start. When a background/worker turn kept the parent turn open, a later human user message could land before the parent emitted more assistant/work output. The parent Worked for... row could then appear above that user message while containing later turn content.

Validation

• pnpm exec turbo run test --filter=@bb/thread-view
• pnpm exec turbo run typecheck --filter=@bb/thread-view