review: one fold implementation, uniform end-edge absence, partial text in tool errors, snapshot scenario

Address ds-review-bot on #2127:

- assistant-output: the rule has ONE implementation, the incremental
  AssistantOutputFold (push/pushText/collect); finalAssistantOutput folds a
  complete suffix, the SDK backend folds notification events, and the ACP
  backend folds raw chunk text into the same streamed fallback.
- subagent/end.lastAssistantMessage: 'no output' is encoded once — absent,
  never [], on both lifecycle shapes (observeRun now omits empty output).
- tool-subagent: a non-completed foreground result stays isError but appends
  the child's preserved partial text after the stop-reason headline.
- Authored keyless snapshot scenario subagent-max-tokens-partial pins the
  assembled transcript: the child's committed log carries the usage-only
  empty message and the parent's tool result carries the partial answer.
- Rule-boundary sentence (message wins over later streamed text) and the
  consumer half recorded in the Agent Note; comments trimmed to pointers.

packages/subagent/tool-subagent/src/index.ts

@@ -134,6 +134,21 @@ function stopReasonError(result: SubagentResult): string | undefined {
   }
 }
 
+/**
+ * Append the child's preserved partial answer to a stop-reason error so a
+ * truncated or cancelled child's real text still reaches the parent model.
+ * @param error - the stop-reason headline.
+ * @param output - the child's selected output (`SubagentResult.output`).
+ * @returns the headline, extended with the partial text when any exists.
+ */
+function withPartialText(error: string, output: ContentBlock[]): string {
+  const text = output
+    .filter((block): block is Extract<ContentBlock, { type: 'text' }> => block.type === 'text')
+    .map(block => block.text)
+    .join('')
+  return text.length === 0 ? error : `${error}\nPartial output before the run ended:\n${text}`
+}
+
 type ForegroundToolResult = {
   readonly kind: 'foreground'
   readonly runId: SubagentRun['id']
@@ -149,8 +164,9 @@ async function settleForegroundRun(run: SubagentRun): Promise<ForegroundToolResu
     run.result.then((result): ForegroundToolResult => {
       const error = stopReasonError(result)
       if (error !== undefined) {
-        // The registry converts this throw to isError; partial output is not success.
-        throw new Error(error)
+        // The registry converts this throw to isError; partial output is not
+        // success, but the preserved partial answer still reaches the parent.
+        throw new Error(withPartialText(error, result.output))
       }
       return {
         kind: 'foreground',