Stage→stage handoff state produced by stages and consumed by later
stages (as opposed to config / options which are pipeline inputs,
or artifacts which has its own registry). The changelog stage
writes here, the release stage reads here.
Rollback policy after the publish stage. BestEffort is the default when
pre-flight ran clean; None is the implicit default otherwise (callers
should warn that rollback is disabled). The CLI flag --rollback=<v>
sets ContextOptions::rollback_mode to Some(v) to override the
default-resolution at the dispatch site.