[codex] Preserve detached desktop action causes - #3371

           Two new classes used at separate call sites. so distinct tags are appropri. Each has variable context (reason/action) interpolated into the message. No redundant singleton discriminator. cause preserved. Imports are namespace from subpaths.there's a potential concern: `DesktopLifecycleRelaunchError` has `reason: Schema.String`. Is `reason` actually a single fixed value at the only call site? If only one call site passes one literal `