magpie-release-prepare

Draft release preparation artefacts for <upstream>: the planning issue, the version-bump and changelog prep PR, or the post-release development-version bump PR. Reads release metadata from <project-config>/release-trains.md and <project-config>/release-management-config.md. Every output is a draft confirmed by the Release Manager before filing; the agent never marks a PR ready, never merges, and never closes any…