fix(ralph-loop): ignore synthetic idle replays

src/hooks/ralph-loop/completion-promise-detector.test.ts

@@ -58,6 +58,29 @@ describe("detectCompletionInSessionMessages", () => {
       // #then
       expect(detected).toBe(true)
     })
+
+    test("#when sinceMessageIndex equals current message count #then should NOT rescan old DONE", async () => {
+      // #given
+      const messages = [
+        {
+          info: { role: "assistant" },
+          parts: [{ type: "text", text: "Old completion <promise>DONE</promise>" }],
+        },
+      ]
+      const ctx = createPluginInput(messages)
+
+      // #when
+      const detected = await detectCompletionInSessionMessages(ctx, {
+        sessionID: "session-123",
+        promise: "DONE",
+        apiTimeoutMs: 1000,
+        directory: "/tmp",
+        sinceMessageIndex: messages.length,
+      })
+
+      // #then
+      expect(detected).toBe(false)
+    })
   })
 
   describe("#given no sinceMessageIndex (backward compat)", () => {