cleanup: terrors OneOf for nested error unions

[joeldsouzax]

Replace the hand-rolled SendError/RemoteSendError flatten() methods (src/error.rs:195,1017, ~66 LOC) with terrors::OneOf, matching nexus/agency. BREAKING signature change across call sites — schedule deliberately. Lower priority than thiserror.

[joeldsouzax]

Folded into #113 (core-rebuild epic #122) as acceptance criteria — closing to avoid duplication. Reopen if you want it tracked standalone.