fix(subagent): keep output past an empty terminal message with one selection rule

A max-tokens step that assembled only tool-call blocks appends an
EMPTY-content assistant/message (the usage host). Three consumers each
hand-rolled output selection and all let it erase the child's real
answer: the in-process readResult and the Activation subagent/end
capture took the last message unfiltered, and the SDK backend let any
message beat its streamed-text fallback; the in-process driver also had
no streamed-text fallback for cancelled turns.

dsh-subagent now owns the canonical rule in src/assistant-output.ts
(last non-empty assistant message, else the accumulated text-delta
stream) and all three consumers apply it. Regression tests in all three
packages fail under the previous selections.

Closes #1514

docs/event-producer-consumer.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/event-producer-consumer.md
-event-producer-consumer.md: 11eecf81a4eccadf2b97026154a78e4ed8a72164
-event-producer-consumer.zh.md: 2db5e596465b4adaf98c1b05692b61de3ced47b9
+event-producer-consumer.md: 1849bf5bd4df5e58370b1eaf5846f14a604fa4f6
+event-producer-consumer.zh.md: f280c50076fa2799e52556aa31a6d2316c151e17