tdd

采用测试驱动开发流程，用测试先行指导功能实现并保证回归安全。

用红-绿-重构（red→green→refactor）的 TDD 循环来先写测试再实现功能，确保每次改动都有对应的回归保护。适合新特性开发或修复 bug 时想要测试优先、保持设计可维护性的场景。还会指导什么是好测试、哪些是反模式、测试放在哪儿以及如何做集成测试，帮助把测试留在代码库里。

# 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