docs: add BDD/Gherkin implementation plan (10 tasks)

Tasks 1-7: Core runner + step definitions (engine, mock, HTTP, pipeline, assertions, state)
Task 8: Pipeline + scenario coverage
Task 9: Strict mode
Task 10: wfctl CLI integration + docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: intel352