docs: add write-e2e-cases skill

.agents/skills/write-e2e-cases/SKILL.md

@@ -0,0 +1,30 @@
+---
+name: write-e2e-cases
+description: Use when adding or updating Rsbuild end-to-end tests in `e2e/cases`, including new feature coverage, bug reproduction, and regression prevention.
+---
+
+# Write E2E Cases
+
+## Steps
+
+1. Review uncommitted git changes to define test scope and target behavior.
+
+2. Read `e2e/README.md` and follow its conventions.
+
+3. Prefer `@e2e/helper` methods (for example `dev`, `build`) to keep tests minimal.
+
+4. Add Playwright cases under `e2e/cases`, following existing directory patterns.
+
+5. Keep assertions focused and readable; avoid redundant setup and checks.
+
+6. Run `pnpm e2e` to validate.
+
+## Case Structure
+
+- Include a `src` directory in every case (required).
+- Add `rsbuild.config.ts` only when needed.
+- Split into multiple case directories when cases need different `src` code or different Rsbuild configs.
+
+## Constraints
+
+- If tests can pass only after source-code changes, do not change source code directly. Explain the required source change and ask the user before proceeding.

e2e/README.md

@@ -37,3 +37,5 @@ test('test 1 + 1', () => {
   expect(1 + 1).toBe(2);
 });
 ```
+
+You can use the local skill at [`write-e2e-cases`](../.agents/skills/write-e2e-cases/SKILL.md) to add new test cases.