feat(KB-617): add Changes tab to task detail modal for viewing file diffs

- Add baseCommitSha field to tasks for diff computation baseline
- Track modified files in executor and store in database
- Add /api/tasks/:id/diff endpoint to serve unified diffs
- Add getTaskDiff() API client function in dashboard
- Create TaskChangesTab component with file list and diff viewer
- Integrate Changes tab into TaskDetailModal with proper styling
- Add changeset for the new diff viewer feature

packages/dashboard/app/api.ts

@@ -1868,3 +1868,14 @@ export function fetchProjectTasks(projectId: string, limit?: number, offset?: nu
 export function fetchProjectConfig(projectId: string): Promise<{ maxConcurrent: number; rootDir: string }> {
   return api<{ maxConcurrent: number; rootDir: string }>(`/projects/${encodeURIComponent(projectId)}/config`);
 }
+
+/** Diff information for a task */
+export interface TaskDiff {
+  files: string[];
+  diffs: Record<string, { stat: string; patch: string }>;
+}
+
+/** Fetch diff information for a task */
+export function fetchTaskDiff(taskId: string): Promise<TaskDiff> {
+  return api<TaskDiff>(`/tasks/${encodeURIComponent(taskId)}/diff`);
+}