Add plan bounce feature that allows plans to be refined through an external
script between plan-checker approval and requirements coverage gate. Activated
via --bounce flag or workflow.plan_bounce config. Includes backup/restore
safety (pre-bounce.md), YAML frontmatter validation, and checker re-run on
bounced plans.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
