tdd

在开发循环中引导红-绿-重构的测试驱动开发实践与规则。

把红-绿-重构的 TDD 循环当作可执行指南来用，说明什么是有价值的测试、测试应该放在哪儿、常见反模式和循环规则。适合想先写测试再实现新功能或修 bug 的场景，也适用于集成测试需求。特点是每个循环阶段都有准则，建议在循环前中后都参考这些规则，别事后才补测试。

# Test-Driven Development

TDD is the red → green loop. This skill is the reference that makes that loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle — consult them before and during the loop, not after.

When exploring the codebase, read `CONTEXT.md` (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.

## What a good test is

Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification — "user can checkout wi