Add missing CLAUDE.md and apply rules to Butil source code (#13118) - #13120

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The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.