docs: add branch creation instruction to git workflow

Explicitly documents the requirement to create a new branch before starting
work from a plan, consistent with the parent workspace CLAUDE.md instruction.

CLAUDE.md

@@ -83,6 +83,11 @@ Each layer depends only on layers below it.
 
 ## Git Workflow
 
+### Before Starting Work
+
+When starting work from a plan, create a new branch matching the plan's scope before
+making any changes. Do not reuse an existing branch from previous work, even if related.
+
 ### Commits
 
 - Format: `type(scope): description`