feat: improve tracing completeness

[deestax]

Summary

• capture and safely redact the assembled provider request while keeping message input queryable
• correlate in-process and deferred subagent turns through explicit trace-parent context
• trace Hindsight retrieval and injection events, including verified memory linkage
• avoid duplicate aggregate subagent usage when child generations are already linked

Validation

• npm run typecheck
• npm test

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[deestax]

Thanks for reviewing this direction. We have decided to use the official extension without carrying a custom tracing-completeness fork. Recent upstream changes also overlap substantially with this branch, so I am closing the PR rather than maintaining the conflicting implementation. No action is needed from the maintainers.