Capability rejections surfaced their internal diagnostic string, so a
checkpoint revert on a provider without rollback support showed 'pi
cannot satisfy rollback_snapshot for command <uuid>: rollback must
return a providerInstanceId thread snapshot'. userFacingDispatchErrorMessage
now renders known policy rejections as provider-named prose ('Pi did not
report its rewound conversation state, so the checkpoint was not
restored.') and keeps the diagnostic form for logs and unknown codes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>