Complete E14-T4 validation coverage and docs

[dmoliveira]

Summary

• expand /start-work validation and recovery selftests to cover missing frontmatter, out-of-order ordinals, invalid in-progress recovery state, and deterministic re-run recovery
• add instructions/plan_execution_workflows.md with sample artifacts and direct/background/recovery workflows for plan execution
• mark Task 14.4 complete, close Epic 14 in roadmap tracking, and update README/changelog references for the new validation workflow guide

Testing

• make validate
• make selftest
• make install-test