feat(cli): redesign implementor UI with card grid layout

- Responsive card grid: 1-4 columns based on terminal width - Smart
column selection for balanced grids (e.g., 4 items = 2x2 not 3+1) -
Expandable cards with "show trace" button for detailed timeline - File
snippets showing first diff line instead of just file names - Rounded
dashed borders for visual distinction from applied changes - Flex-based
card sizing using flexGrow for proper width handling - Added
computeSmartColumns helper with unit tests - Added getEditedFileSnippets
helper for content previews

Author: brandonkachen