fix(api): shim terminal joins the transcript; sessions tolerate delta-less results

The prompt-shim terminal returned without pushing its final assistant
text onto the conversation array, so shim-path transcript deltas ended
on a tool result. Sessions now fall back to the minimal user/assistant
pair when a result carries no transcriptDelta, keeping the pre-0.10
history shape for legacy generateText wrappers and test doubles.

Author: jddunn

src/api/agent.ts

@@ -1001,9 +1001,15 @@ export function agent(opts: AgentOptions): Agent {
             }
           }
 
-          if (historyBuffer && result.transcriptDelta) {
+          if (historyBuffer) {
+            // Lossless delta when the call carried one; minimal user/assistant
+            // pair otherwise (legacy generateText wrappers and test doubles
+            // predating transcriptDelta keep the pre-0.10 history shape).
             historyBuffer.appendSendDelta(
-              result.transcriptDelta,
+              result.transcriptDelta ?? [
+                userMessage as unknown as SessionTranscriptMessage,
+                { role: 'assistant', content: result.text },
+              ],
               sendOpts?.blockLabel,
               epochAtStart,
             );

src/api/generateText.ts

@@ -1700,6 +1700,9 @@ export async function generateText(opts: GenerateTextOptions): Promise<GenerateT
           surface: 'generateText',
           durationMs: Date.now() - rootStartedAt,
         });
+        // The shim's final assistant text lives in loopResult, never on the
+        // messages array — push it so the transcript delta ends on the reply.
+        messages.push({ role: 'assistant', content: loopResult.text });
         return {
           transcriptDelta: messages.slice(transcriptDeltaStart) as unknown as SessionTranscriptMessage[],
           provider: resolved.providerId,