CHW-1408 BillyActionFailed

Default message: Billy action could not be applied; inspect the request-specific error message for the next step
HTTP status: 400 (client)

Problem. A Billy action (e.g. user.disable) was rejected or failed mid-flight. Cause. The handler dispatched the proposal but the typed billyActionError envelope reported a non-success outcome (invalid argument, missing target, downstream failure). The body is preserved verbatim as {"error":"<message>"} so existing Billy clients keep parsing the original shape. Fix. Inspect the request-specific error message for the next step — typically supply a missing field, correct the target id, or retry once the downstream condition clears. HTTP varies (400/403/404/500) per the typed error.