Add PlanApplyService with diff(), artifacts(), persist_apply_summary(),
handle_merge_failure(), and guard_empty_changeset() methods.
- plan diff: renders changeset in rich/plain/json/yaml formats
- plan artifacts: shows plan metadata, changeset summary, sandbox refs
- persist_apply_summary: stores files_changed/validations_run in plan metadata
- handle_merge_failure: transitions plan to error state with conflict details
- guard_empty_changeset: blocks apply on empty changeset (--allow-empty override)
CLI: plan diff and plan artifacts commands with --format flag.
Tests: 20 Behave scenarios, 8 Robot integration tests, 4 ASV benchmarks.
Docs: docs/reference/plan_apply.md with CLI reference.
Implements D0b.apply from the implementation plan.
freemo