Validate

When the user asks whether sprint-status.yaml is well-formed, run:

uv run {skill-root}/scripts/sprint_plan.py validate \
  --status-file {implementation_artifacts}/sprint-status.yaml

Never writes; exits 0 whether valid or not. Report valid in one line. If problems is non-empty, list them plainly (each names the key or field at fault) and offer the fix flow (fix-sprint-status.md). If legacy_mapped is non-empty, note the file still uses v6 status names and that any regenerate will rewrite them to the modern vocabulary — progress is preserved either way.