docs(collab): add atomic commits rule to commit message guidelines

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

COLLABORATING.md

@@ -122,6 +122,24 @@ Closes #7
 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
 ```
 
+### Atomic commits
+
+Each commit must do exactly one logical thing. Never bundle multiple unrelated changes into a single commit, even if they are small.
+
+**Wrong** — three changes in one commit:
+```
+fix(e2e+i18n): add missing DE translation, fix test selectors, fix lang switching
+```
+
+**Right** — three separate commits:
+```
+fix(i18n): add missing person_btn_conversations DE translation
+fix(e2e): exclude /persons/new from person link selector
+fix(e2e): clear locale cookie when switching back to base language
+```
+
+When in doubt, commit more often rather than less.
+
 ## Code Style Reminders
 
 - Pure functions over stateful helpers where possible