Write and maintain Gherkin `.feature` behavior specifications (behavior specs) for features and core system modules. Use AFTER completing requirement clarification and writing-plans (docs/plan ready), BEFORE implementation begins. Triggers writing behavior specs, defining acceptance criteria, updating system behavior definitions, creating feature specs, reviewing core module behavior changes, Gherkin specs, BDD specs, 行为规范, 行为定义, 验收标准, 系统行为. Outputs `.feature` files to docs/spec/feature-spec/ and docs/spec/core-spec/ and lints generated specs with gherkin-lint. Also serves as agent memory for understanding current system behavior across modules.