fix(mcp): relay terminal provider helper events #52

PR #27's inbox relay currently recognizes terminal run.updated events but skips terminal subagent.updated events carrying the bound runId. Native webhook observeEvent still maps helper completion/failure to thread.run.result/thread.blocker. Hosted review PRRT_kwDOVfiJHc62iU96 classifies this as P2.

Forward matching helper occurrences while preserving their helper identity. A helper must not consume the parent run's outcome deduplication slot or expire its binding. Add a focused helper-plus-parent-outcome case. Reuse the existing relay tables; no new event migration is allocated.

Tracked under Chris's nonblocking P2 follow-up rule. The core acceptance remains a scratch run outcome relayed through the genuine inbox watch. No service update.