The four mut-local CheckError variants (MutAssignOutOfScope,
AssignTypeMismatch, UnsupportedMutVarType, MutVarCapturedByLambda)
embedded the bracketed kebab code in their thiserror Display body,
while the non-JSON CLI formatter independently prepends [code] from
CheckError::code() — doubling it. Non-mut variants never embedded
the bracket; this brings the four in line with that convention.
RED-first via the debug skill: ct1_check_cli.rs pins the observable
property (code appears exactly once in the rendered human
diagnostic). GREEN was a trivial four-string mechanical edit.
Tests 598 -> 599.
