fix: required background completion silently disappears (#120453)

* fix(agents): surface silent required completions

* refactor(agents): simplify completion fallback handling

* fix(agents): require visible completion delivery

* fix(agents): preserve resolved session patches

* fix(agents): bound resolved session patch output

* fix(agents): reject silent automatic completions

* fix(agents): settle committed completion side effects

* fix(agents): retain ultra thinking profile

* fix(agents): require destination-safe completion evidence

* fix(agents): block invisible completion side effects

* fix(agents): preserve outbound no-replay evidence

* fix(agents): preserve resolved session identifiers

docs/tools/subagents.md

@@ -528,8 +528,9 @@ fallbacks. Fully isolated auth per agent is not supported yet.
 Sub-agents report back via an announce step:
 
 - The announce step runs inside the sub-agent session (not the requester session).
-- If the sub-agent replies exactly `ANNOUNCE_SKIP`, nothing is posted.
-- If the latest assistant text is the exact silent token `NO_REPLY` / `no_reply`, announce output is suppressed even if earlier visible progress existed.
+- An exact `ANNOUNCE_SKIP` response suppresses announce output.
+- For completion-required runs, an exact child `NO_REPLY` response or no output is a missing deliverable handed to the requester/parent for visible representation or retry; it is not credited as silent delivery.
+- Optional, duplicate, already-visible, or otherwise non-required paths may use exact `NO_REPLY` for intentional silence.
 
 Delivery depends on requester depth: