fix: resolve 8 bugs affecting real users

- Fix KeyError 'root' in FileGrouper when merging groups with no root-level files
- Fix changelog commit parsing breaking on multiline bodies (use %x00 separator)
- Add empty choices/content checks in OpenAI, Anthropic, OpenRouter providers
- Fix config edit crash when config is empty
- Fix validate command using wrong color for 'no staged files'
- Add type check for corrupted YAML config (non-dict returns empty dict)

devgen/modules/diff_builder.py

@@ -129,10 +129,7 @@ class FileGrouper:
             parent_path = str(Path(deepest).parent)
             new_key = "root" if parent_path == "." else parent_path
             files = groups.pop(deepest)
-            if new_key in groups:
-                groups[new_key].extend(files)
-            else:
-                groups["root"].extend(files)
+            groups.setdefault(new_key, []).extend(files)
         return groups