fix: update scheduler filesystem path from .kb to .fusion for task validation
style: clean up TaskDetailModal styling with reusable CSS classes
test: add comprehensive tests for file-service operations with mocked filesystem
- 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
