- Create `codewalker` module with file system utilities
- Refactor `Workspace` file operations to use `codewalker`
- Add `include_tree` flag to `HeropromptChild` struct
- Implement new `/selection` API endpoint for workspace
- Sync frontend selection state to backend via new API
- Introduce HeropromptChild to unify file and dir items
- Replace nested Dir/File structs with a flat `children` list
- Generate prompt content by traversing the filesystem on-demand
- Add `workspace.add_file` for direct file selection
- Simplify `workspace.add_dir` to only add the directory path