feat(FN-3606): correct chat-bound message routing and stabilize merger auto

Merger receives substantial hardening: autostash race-rescue with de-duplication, advisory observer for destructive operations, and subject-line preference for step headlines. The TUI gains a narrow log-split mode on System panel with proper back-navigation to the main pane. Chat header and mobile n

Fusion-Task-Id: FN-3606

packages/engine/src/__tests__/agent-tools.test.ts

@@ -716,6 +716,23 @@ describe("createSendMessageTool", () => {
     );
   });
 
+  it.each(["dashboard", "user:dashboard", "User: user:dashboard"])(
+    "infers agent-to-user when type is omitted for dashboard alias '%s'",
+    async (dashboardAlias) => {
+      const mockMessage = createMessage({ toId: "dashboard", toType: "user", type: "agent-to-user" });
+      vi.mocked(messageStore.sendMessage).mockReturnValue(mockMessage);
+
+      await executeTool(tool, {
+        to_id: dashboardAlias,
+        content: "Test",
+      });
+
+      expect(messageStore.sendMessage).toHaveBeenCalledWith(
+        expect.objectContaining({ toId: "dashboard", toType: "user", type: "agent-to-user" }),
+      );
+    },
+  );
+
   it("uses provided type when specified and maps recipient type for agent-to-user", async () => {
     const mockMessage = createMessage({ toType: "user", type: "agent-to-user" });
     vi.mocked(messageStore.sendMessage).mockReturnValue(mockMessage);

packages/engine/src/agent-tools.ts

@@ -1431,7 +1431,9 @@ export function createSendMessageTool(messageStore: MessageStore, fromAgentId: s
       }
 
       try {
-        const messageType = params.type ?? "agent-to-agent";
+        const inferredDashboardRecipient = normalizeMessageParticipant(params.to_id, "user");
+        const messageType = params.type
+          ?? (inferredDashboardRecipient.id === DASHBOARD_USER_ID ? "agent-to-user" : "agent-to-agent");
         const recipientType: "user" | "agent" = messageType === "agent-to-user" ? "user" : "agent";
         const recipient = recipientType === "user"
           ? normalizeMessageParticipant(params.to_id, recipientType)