test(subagent): rewrite continuation spec for activation lifecycle
Covers the inbox-acceptance return boundary, pre-acceptance rollback, caller-signal ownership on both sides of acceptance, residency-only routing, single-inbox FIFO ordering across parent and user origins, waiting wakeup with a retained handle, cold resume without the historical parent, ownership registration and release, child-first disposal, send-versus-dispose races, durability failure without an ownership leak, per-epoch lifecycle pairing, and the absence of cancellation, steering, and report surfaces.
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