fix(signal): land #31138 syncMessage presence filtering (@Sid-Qin)

Landed from contributor PR #31138 by @Sid-Qin.

Co-authored-by: Sid-Qin <sidqin0410@gmail.com>

src/signal/monitor/event-handler.inbound-contract.test.ts

@@ -201,4 +201,29 @@ describe("signal createSignalEventHandler inbound contract", () => {
     expect(capture.ctx).toBeUndefined();
     expect(dispatchInboundMessageMock).not.toHaveBeenCalled();
   });
+
+  it("drops sync envelopes when syncMessage is present but null", async () => {
+    const handler = createSignalEventHandler(
+      createBaseSignalEventHandlerDeps({
+        cfg: {
+          messages: { inbound: { debounceMs: 0 } },
+          channels: { signal: { dmPolicy: "open", allowFrom: ["*"] } },
+        },
+        historyLimit: 0,
+      }),
+    );
+
+    await handler(
+      createSignalReceiveEvent({
+        syncMessage: null,
+        dataMessage: {
+          message: "replayed sentTranscript envelope",
+          attachments: [],
+        },
+      }),
+    );
+
+    expect(capture.ctx).toBeUndefined();
+    expect(dispatchInboundMessageMock).not.toHaveBeenCalled();
+  });
 });

src/signal/monitor/event-handler.ts

@@ -438,9 +438,11 @@ export function createSignalEventHandler(deps: SignalEventHandlerDeps) {
       return;
     }
 
-    // For non-own sync messages (e.g., messages synced from other devices),
-    // we could process them but for now we skip to be conservative
-    if (envelope.syncMessage) {
+    // Filter all sync messages (sentTranscript, readReceipts, etc.).
+    // signal-cli may set syncMessage to null instead of omitting it, so
+    // check property existence rather than truthiness to avoid replaying
+    // the bot's own sent messages on daemon restart.
+    if ("syncMessage" in envelope) {
       return;
     }