feat(KB-651): add changed files diff viewer for tasks

- Add backend API endpoint to get changed files and diffs for a task worktree
- Add useChangedFiles hook for fetching and managing diff data
- Create ChangedFilesModal component with file list and diff viewer
- Integrate modal into TaskCard with click handler to view changes
- Add frontend API integration and type definitions
- Include unit tests for hook and modal components
- Add changeset for the new feature
- Remove deprecated mission-related test files

packages/dashboard/app/components/WorktreeGroup.tsx

@@ -15,7 +15,7 @@ interface WorktreeGroupProps {
     id: string,
     updates: { title?: string; description?: string; dependencies?: string[] }
   ) => Promise<Task>;
-  onOpenFilesForTask?: (taskId: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
 }
 
 function WorktreeGroupComponent({