Sync Sprint Status

Set development_status[{story_key}] to {target_status} in {{.implementation_artifacts}}/sprint-status.yaml. If {story_key} is missing, warn once and stop. If the story is already at {target_status} or later, stop. When {target_status} is in-progress, set parent epic (e.g. 3-2-foo → epic-3) from backlog to in-progress if present. Update last_updated. Preserve comments and structure.