feat(FN-926): filter out files with empty diffs from file-diffs endpoint

- Filter files where diff body is empty (e.g., binary files, unchanged) from the /api/tasks/:id/file-diffs response
- Add tests verifying empty diffs are excluded while non-empty diffs are preserved
- Reduces noise in the dashboard diff viewer by omitting irrelevant file entries

packages/dashboard/src/__tests__/routes-file-diffs.test.ts

@@ -511,6 +511,36 @@ describe("GET /api/tasks/:id/file-diffs", () => {
     expect(paths.sort()).toEqual(["src/d.ts", "src/e.ts"]);
   });
 
+  it("filters out files with empty diffs (mode-only changes, binary files)", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ baseBranch: "main", baseCommitSha: "taskbase456" }));
+
+    mockExecSync.mockImplementation((command) => {
+      const cmd = String(command);
+      if (cmd === "git merge-base --is-ancestor taskbase456 HEAD") {
+        return "" as any;
+      }
+      // git reports the file as modified
+      if (cmd === "git diff --name-status taskbase456..HEAD") {
+        return "M\tsrc/mode-only.txt\n" as any;
+      }
+      if (cmd === "git diff --name-status") {
+        return "" as any;
+      }
+      // but the diff is empty (mode-only change)
+      if (cmd === 'git diff taskbase456..HEAD -- "src/mode-only.txt"') {
+        return "" as any;
+      }
+      throw new Error(`Unexpected command: ${cmd}`);
+    });
+
+    const response = await requestFileDiffs(store);
+
+    expect(response.status).toBe(200);
+    // File should be filtered out because diff is empty
+    expect(response.body).toHaveLength(0);
+  });
+
   it("agrees with session-files under task-scoped base resolution", async () => {
     const store = new MockStore();
     // Both routes should produce the same file list when baseCommitSha is valid

packages/dashboard/src/routes.ts

@@ -7577,7 +7577,7 @@ Output ONLY the prompt text (no markdown, no explanations).`;
       // against the resolved merge-base.
       const diffRange = diffBase ? `${diffBase}..HEAD` : "HEAD";
 
-      const files = Array.from(fileMap.entries()).map(([filePath, { statusCode, oldPath }]) => {
+      const files = Array.from(fileMap.entries()).flatMap(([filePath, { statusCode, oldPath }]) => {
         let status: "added" | "modified" | "deleted" | "renamed" = "modified";
 
         if (statusCode.startsWith("A")) {
@@ -7599,7 +7599,12 @@ Output ONLY the prompt text (no markdown, no explanations).`;
           diff = "";
         }
 
-        return oldPath ? { path: filePath, status, diff, oldPath } : { path: filePath, status, diff };
+        // Filter out files with empty diffs (mode-only changes, binary files, etc.)
+        if (!diff) {
+          return [];
+        }
+
+        return oldPath ? [{ path: filePath, status, diff, oldPath }] : [{ path: filePath, status, diff }];
       });
 
       fileDiffsCache.set(task.id, {