docs: distribute package-specific GEMINI.md context to each package (#22734)

packages/test-utils/GEMINI.md

@@ -0,0 +1,16 @@
+# Gemini CLI Test Utils (`@google/gemini-cli-test-utils`)
+
+Shared test utilities used across the monorepo. This is a private package — not
+published to npm.
+
+## Key Modules
+
+- `src/test-rig.ts`: The primary test rig for spinning up end-to-end CLI
+  sessions with mock responses.
+- `src/file-system-test-helpers.ts`: Helpers for creating temporary file system
+  fixtures.
+- `src/mock-utils.ts`: Common mock utilities.
+
+## Usage
+
+Import from `@google/gemini-cli-test-utils` in test files across the monorepo.