feat(FN-906): add file navigation to changes tab and reduce diff margins

- Add file navigation dropdown/keyboard support to the TaskChangesTab component
- Reduce horizontal margins in diff display for better readability
- Add comprehensive tests for file navigation behavior
- Include changeset for patch release

packages/dashboard/app/components/TaskChangesTab.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useCallback } from "react";
-import { FileCode, ChevronDown, ChevronRight, AlertCircle, GitCommit } from "lucide-react";
+import { FileCode, ChevronDown, ChevronRight, ChevronLeft, AlertCircle, GitCommit } from "lucide-react";
 import type { MergeDetails, Column } from "@fusion/core";
 import { fetchTaskDiff, type TaskDiff } from "../api";
 import { highlightDiff } from "../utils/highlightDiff";
@@ -49,6 +49,7 @@ export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetai
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
   const [expandedFiles, setExpandedFiles] = useState<Set<string>>(new Set());
+  const [currentFileIndex, setCurrentFileIndex] = useState<number | null>(null);
 
   const canLoad = column === "in-progress" || column === "in-review" || column === "done";
 
@@ -73,6 +74,7 @@ export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetai
       setStats(data.stats);
       if (normalized.length > 0) {
         setExpandedFiles(new Set([normalized[0].path]));
+        setCurrentFileIndex(0);
       }
     } catch (err: any) {
       setError(err.message || "Failed to load diff");
@@ -92,11 +94,27 @@ export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetai
         next.delete(filePath);
       } else {
         next.add(filePath);
+        // Update currentFileIndex to the newly expanded file
+        const idx = files.findIndex((f) => f.path === filePath);
+        if (idx !== -1) {
+          setCurrentFileIndex(idx);
+        }
       }
       return next;
     });
   };
 
+  const navigateToFile = (index: number) => {
+    if (index < 0 || index >= files.length) return;
+    const targetPath = files[index].path;
+    // Collapse all files and expand only the target
+    setExpandedFiles(new Set([targetPath]));
+    setCurrentFileIndex(index);
+  };
+
+  const canGoPrev = currentFileIndex !== null && currentFileIndex > 0;
+  const canGoNext = currentFileIndex !== null && currentFileIndex < files.length - 1;
+
   if (loading) {
     return (
       <div className="detail-section">
@@ -183,13 +201,40 @@ export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetai
             <span className="diff-del">-{stats.deletions}</span>
           </span>
         </h4>
-        <button
-          className="btn btn-sm"
-          onClick={loadDiff}
-          disabled={loading}
-        >
-          Refresh
-        </button>
+        <div className="changes-header-actions">
+          {files.length > 0 && (
+            <div className="changes-nav">
+              <button
+                className="btn btn-sm btn-icon"
+                onClick={() => canGoPrev && navigateToFile(currentFileIndex! - 1)}
+                disabled={!canGoPrev}
+                title="Previous file"
+                aria-label="Previous file"
+              >
+                <ChevronLeft size={14} />
+              </button>
+              <span className="changes-nav-indicator" aria-live="polite">
+                {currentFileIndex !== null ? `${currentFileIndex + 1}/${files.length}` : `—/${files.length}`}
+              </span>
+              <button
+                className="btn btn-sm btn-icon"
+                onClick={() => canGoNext && navigateToFile(currentFileIndex! + 1)}
+                disabled={!canGoNext}
+                title="Next file"
+                aria-label="Next file"
+              >
+                <ChevronRight size={14} />
+              </button>
+            </div>
+          )}
+          <button
+            className="btn btn-sm"
+            onClick={loadDiff}
+            disabled={loading}
+          >
+            Refresh
+          </button>
+        </div>
       </div>
 
       <div className="changes-file-list">

packages/dashboard/app/components/__tests__/TaskChangesTab.test.tsx

@@ -13,6 +13,7 @@ vi.mock("lucide-react", () => ({
   FileCode: () => null,
   ChevronDown: ({ size }: any) => <span data-testid="chevron-down" />,
   ChevronRight: ({ size }: any) => <span data-testid="chevron-right" />,
+  ChevronLeft: ({ size }: any) => <span data-testid="chevron-left" />,
   AlertCircle: () => null,
   GitCommit: () => null,
 }));
@@ -444,3 +445,176 @@ describe("TaskChangesTab — status-to-class mapping", () => {
     expect(delStat).toBeTruthy();
   });
 });
+
+describe("TaskChangesTab — file navigation", () => {
+  it("renders Previous and Next navigation buttons", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("Files Changed (2)")).toBeTruthy();
+    });
+
+    expect(screen.getByLabelText("Previous file")).toBeTruthy();
+    expect(screen.getByLabelText("Next file")).toBeTruthy();
+  });
+
+  it("shows file position indicator in current/total format", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+  });
+
+  it("disables Previous button on first file", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    expect(screen.getByLabelText("Previous file")).toBeDisabled();
+  });
+
+  it("enables Next button on first file", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    expect(screen.getByLabelText("Next file")).not.toBeDisabled();
+  });
+
+  it("navigates to next file when Next is clicked", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    // Click Next
+    fireEvent.click(screen.getByLabelText("Next file"));
+
+    // Indicator should update to 2/2
+    expect(screen.getByText("2/2")).toBeTruthy();
+  });
+
+  it("disables Next button on last file", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    // Navigate to last file
+    fireEvent.click(screen.getByLabelText("Next file"));
+
+    expect(screen.getByText("2/2")).toBeTruthy();
+    expect(screen.getByLabelText("Next file")).toBeDisabled();
+    expect(screen.getByLabelText("Previous file")).not.toBeDisabled();
+  });
+
+  it("navigates back to previous file when Previous is clicked", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    // Go to next, then back
+    fireEvent.click(screen.getByLabelText("Next file"));
+    expect(screen.getByText("2/2")).toBeTruthy();
+
+    fireEvent.click(screen.getByLabelText("Previous file"));
+    expect(screen.getByText("1/2")).toBeTruthy();
+  });
+
+  it("expands only the navigated file", async () => {
+    mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
+    const { container } = render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1/2")).toBeTruthy();
+    });
+
+    // Initially only first file expanded
+    expect(container.querySelectorAll(".changes-file-content")).toHaveLength(1);
+
+    // Navigate to second file
+    fireEvent.click(screen.getByLabelText("Next file"));
+
+    // Still only one file expanded (the second one)
+    expect(container.querySelectorAll(".changes-file-content")).toHaveLength(1);
+  });
+});

packages/dashboard/app/styles.css

@@ -3458,6 +3458,26 @@ body {
   margin: 0;
 }
 
+.changes-header-actions {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-sm);
+}
+
+.changes-nav {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-xs);
+}
+
+.changes-nav-indicator {
+  font-size: 12px;
+  color: var(--text-muted);
+  font-family: var(--font-mono);
+  min-width: 32px;
+  text-align: center;
+}
+
 .changes-file-list {
   display: flex;
   flex-direction: column;
@@ -3578,7 +3598,7 @@ body {
 
 .changes-diff-patch {
   margin: 0;
-  padding: var(--space-sm) var(--space-md);
+  padding: var(--space-sm) 0;
   background: rgba(0, 0, 0, 0.14);
   font-family: var(--font-mono);
   font-size: 11px;
@@ -15116,7 +15136,7 @@ html .column.drag-over * {
 
 .changes-diff-patch {
   margin: 0;
-  padding: var(--space-md);
+  padding: var(--space-sm) 0;
   font-size: 12px;
   line-height: 1.5;
   overflow-x: auto;