Extract the standalone gateway-event helpers (session-info patch derivation,
completion-error detection, todo-payload routing, delegate_task -> subagent
spec mapping, + the stream-flush/subagent-event constants) out of the
1,285-line hook into a colocated, tested use-message-stream/utils.ts. index.ts
keeps the stateful streaming hook and consumes the helpers.
Pure restructuring, no behaviour change; folder index keeps the import path
intact. index.ts: 1,285 -> ~1,120. Adds unit tests for the pure helpers.
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