refine Claude instructions (#10744)

Author: max-sixty

CLAUDE.md

@@ -22,7 +22,9 @@ uv run dmypy run  # Type checking with mypy
 
 ## GitHub Interaction Guidelines
 
-- **NEVER impersonate the user on GitHub** - Do not post comments, create issues, or interact with the xarray GitHub repository unless explicitly instructed
-- Never create GitHub issues or PRs unless explicitly requested by the user
-- Never post "update" messages, progress reports, or explanatory comments on GitHub issues/PRs unless specifically asked
-- Always require explicit user direction before creating pull requests or pushing to the xarray GitHub repository
+- **NEVER impersonate the user on GitHub**, always sign off with something like
+  "[This is Claude Code on behalf of Jane Doe]"
+- Never create issues nor pull requests on the xarray GitHub repository unless
+  explicitly instructed
+- Never post "update" messages, progress reports, or explanatory comments on
+  GitHub issues/PRs unless specifically instructed