Agent Skills: lisa-sync-down

This skill should be used to run a back-sync of an environment branch DOWN the deploy chain on demand — propagating merges (e.g. hotfixes) from a higher environment to every lower one. Given a source environment name or branch (e.g. `production`), it derives the source→target chain from `.lisa.config.json` `deploy.order` + `deploy.branches`, then for each downward hop creates a sync branch, merges, resolves conflicts, opens or updates a PR, and enables auto-merge. Runnable by a developer locally or by CI.