docs: enhance TDD guidelines in CLAUDE.md and INSTRUCTIONS.md

CLAUDE.md

@@ -6,6 +6,29 @@ right knowledge.
 
 ---
 
+## Default Workflow: Test-Driven Development (TDD)
+
+> **Every implementation step in this project follows TDD. This is non-negotiable.**
+
+The cycle for every step is:
+
+| Phase      | Action                                                                  | Gate                               |
+|------------|-------------------------------------------------------------------------|------------------------------------|
+| **Red**    | Write test file(s) for the step. Run the test suite.                    | New tests must **fail**.           |
+| **Green**  | Write the minimum implementation to make all tests pass.                | All tests must **pass**.           |
+| **Refactor** | Clean up the implementation.                                          | Tests must stay **green**.         |
+| **Done**   | Mark step ✅ only when the full build is green.                         | `./gradlew build` / `npm run build && npm run test` |
+
+**Rules:**
+- Never write implementation code before the test file exists and the tests fail.
+- Never mark a step Done unless the full test suite passes.
+- Test method names (Kotlin/JUnit): `should_[expectedBehavior]_when_[condition]`.
+- Backend mocking: **MockK** only (not Mockito).
+- Backend integration tests: `@SpringBootTest` with **H2 in-memory** database.
+- Frontend tests: **Vitest** + **React Testing Library**, mocked Axios.
+
+---
+
 ## Project Overview
 
 - **Type:** Monorepo (backend + frontend in the same repository)
@@ -361,17 +384,32 @@ docker compose down
 
 ## Testing Guidelines
 
+> **This project follows strict TDD.** For every implementation step, tests are written
+> first (Red), then the implementation is added until all tests pass (Green), then code is
+> cleaned up (Refactor). Never write implementation code before the tests exist and fail.
+
+### TDD Workflow (apply to every step)
+1. **Red** — Write the test file(s). Run `./gradlew test` (backend) or `npm run test`
+   (frontend) and confirm the new tests **fail** (compile errors are acceptable at this stage).
+2. **Green** — Write the minimum implementation needed to make all tests pass.
+3. **Refactor** — Clean up the implementation while keeping tests green.
+4. A step is only ✅ Done when `./gradlew build` (backend) or
+   `npm run build && npm run test` (frontend) is fully green.
+
 ### Backend
-- Every service class must have a corresponding unit test class.
+- Every service class must have a corresponding unit test class **written before the service**.
 - Use **MockK** for mocking (not Mockito).
 - Integration tests use `@SpringBootTest` and an **H2 in-memory** database.
 - Test method names follow: `should_[expectedBehavior]_when_[condition]`.
 - Minimum 80% code coverage for service classes.
+- Test files live in `src/test/kotlin/` mirroring the `src/main/kotlin/` package structure.
 
 ### Frontend
-- Every page component must have at least a smoke test (renders without crashing).
+- Every page component must have at least a smoke test (renders without crashing),
+  **written before the component**.
 - API layer functions must be tested with mocked Axios responses.
 - Use **Vitest** as the test runner and **React Testing Library** for component tests.
+- Test files live in `src/__tests__/` mirroring the `src/` structure.
 
 ---
 
@@ -458,6 +496,8 @@ BODY:
 - Branch naming: `feature/<short-description>`, `fix/<short-description>`, `chore/<short-description>`
 - Commit messages follow [Conventional Commits](https://www.conventionalcommits.org/): `feat:`, `fix:`, `chore:`, `docs:`, `test:`
 - Scope your commits: `feat(backend):`, `feat(frontend):`, `chore(docker):`
+- **TDD commit order per step:** first `test(<scope>): add failing tests for <step>`, then
+  `feat(<scope>): implement <step> — all tests passing`.
 - PRs require all CI checks to pass before merging.
 - Never commit directly to `main`.