Docs: Enhance "Writing Tests" section in GEMINI.md (#688)

GEMINI.md

@@ -16,7 +16,47 @@ npm run preflight
 
 ## Writing Tests
 
-When writing tests look at existing tests in packages/core and packages/cli to conform with the conventions in those files.
+This project uses **Vitest** as its primary testing framework. When writing tests, aim to follow existing patterns. Key conventions include:
+
+### Test Structure and Framework
+
+- **Framework**: All tests are written using Vitest (`describe`, `it`, `expect`, `vi`).
+- **File Location**: Test files (`*.test.ts` for logic, `*.test.tsx` for React components) are co-located with the source files they test.
+- **Configuration**: Test environments are defined in `vitest.config.ts` files.
+- **Setup/Teardown**: Use `beforeEach` and `afterEach`. Commonly, `vi.resetAllMocks()` is called in `beforeEach` and `vi.restoreAllMocks()` in `afterEach`.
+
+### Mocking (`vi` from Vitest)
+
+- **ES Modules**: Mock with `vi.mock('module-name', async (importOriginal) => { ... })`. Use `importOriginal` for selective mocking.
+  - _Example_: `vi.mock('os', async (importOriginal) => { const actual = await importOriginal(); return { ...actual, homedir: vi.fn() }; });`
+- **Mocking Order**: For critical dependencies (e.g., `os`, `fs`) that affect module-level constants, place `vi.mock` at the _very top_ of the test file, before other imports.
+- **Hoisting**: Use `const myMock = vi.hoisted(() => vi.fn());` if a mock function needs to be defined before its use in a `vi.mock` factory.
+- **Mock Functions**: Create with `vi.fn()`. Define behavior with `mockImplementation()`, `mockResolvedValue()`, or `mockRejectedValue()`.
+- **Spying**: Use `vi.spyOn(object, 'methodName')`. Restore spies with `mockRestore()` in `afterEach`.
+
+### Commonly Mocked Modules
+
+- **Node.js built-ins**: `fs`, `fs/promises`, `os` (especially `os.homedir()`), `path`, `child_process` (`execSync`, `spawn`).
+- **External SDKs**: `@google/genai`, `@modelcontextprotocol/sdk`.
+- **Internal Project Modules**: Dependencies from other project packages are often mocked.
+
+### React Component Testing (CLI UI - Ink)
+
+- Use `render()` from `ink-testing-library`.
+- Assert output with `lastFrame()`.
+- Wrap components in necessary `Context.Provider`s.
+- Mock custom React hooks and complex child components using `vi.mock()`.
+
+### Asynchronous Testing
+
+- Use `async/await`.
+- For timers, use `vi.useFakeTimers()`, `vi.advanceTimersByTimeAsync()`, `vi.runAllTimersAsync()`.
+- Test promise rejections with `await expect(promise).rejects.toThrow(...)`.
+
+### General Guidance
+
+- When adding tests, first examine existing tests to understand and conform to established conventions.
+- Pay close attention to the mocks at the top of existing test files; they reveal critical dependencies and how they are managed in a test environment.
 
 ## Git Repo