docs: Add Jules.md for AI agent (#7303)

The instruction document was generated by Jules and should provide
information useful for the agent to work on our repository.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

agents.md

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+# Agents
+
+This guide provides essential information for working within the `firebase-android-sdk` repository.
+
+## Environment Setup
+
+To work with this repository, the Android SDK must be installed. Use the `sdkmanager` command-line
+tool for this purpose.
+
+1.  **Install Android SDK Command-Line Tools**:
+
+    - If not already installed, download the command-line tools from the
+      [Android Studio page](https://developer.android.com/studio#command-line-tools-only).
+    - Create a directory for the Android SDK, e.g., `android_sdk`.
+    - Unzip the downloaded package. This will create a `cmdline-tools` directory. Move this
+      directory to `android_sdk/cmdline-tools/latest`.
+    - The final structure should be `android_sdk/cmdline-tools/latest/`.
+
+2.  **Install required SDK packages**:
+
+    - Use `sdkmanager` to install the necessary platforms, build tools, and other packages. For
+      example:
+
+      ```bash
+      # List all available packages
+      sdkmanager --list
+
+      # Install platform tools and the SDK for API level 33
+      sdkmanager "platform-tools" "platforms;android-33"
+
+      # Accept all licenses
+      yes | sdkmanager --licenses
+      ```
+
+    - Refer to the specific requirements of the project to determine which packages to install.
+
+3.  **Configure for integration tests**:
+
+    - To run integration tests, a `google-services.json` file is required.
+    - Place this file in the root of the repository.
+
+4.  **Install NDK for specific projects**:
+    - Some projects, like `firebase-crashlytics-ndk`, require a specific version of the Android NDK.
+      You can install it using `sdkmanager`. For example, to install NDK version 21.4.7075529, you
+      would run `sdkmanager "ndk;21.4.7075529"`. Always refer to the project's `README.md` for the
+      exact version required.
+
+---
+
+## Testing
+
+This repository uses two main types of tests:
+
+1.  **Unit Tests**:
+
+    - These tests run on the local JVM.
+    - To execute unit tests for a specific project, run:
+      ```bash
+      ./gradlew :<firebase-project>:check
+      ```
+
+2.  **Integration Tests**:
+    - These tests run on a hardware device or emulator.
+    - Ensure a `google-services.json` file is present in the repository root.
+    - To execute integration tests for a specific project, run:
+      ```bash
+      ./gradlew :<firebase-project>:connectedCheck
+      ```
+
+---
+
+## API Surface
+
+The public API of the Firebase SDKs is managed using a set of annotations:
+
+- `@PublicApi`: Marks APIs that are intended for public consumption by developers.
+- `@KeepForSdk`: Marks APIs that are intended for use by other Firebase SDKs. These APIs will
+  trigger a linter error if used by developers outside of a Firebase package.
+- `@Keep`: Marks APIs that need to be preserved at runtime, usually due to reflection. This
+  annotation should be used sparingly as it prevents Proguard from removing or renaming the code.
+
+---
+
+## Common Patterns
+
+This repository uses a combination of dependency injection frameworks:
+
+- **`firebase-components`**: This is a custom dependency injection framework used for discovery and
+  dependency injection between different Firebase SDKs. It allows SDKs to register their components
+  and declare dependencies on other components. The initialization is managed by `FirebaseApp`.
+
+- **Dagger**: Dagger is used for internal dependency injection within individual SDKs. This helps to
+  create more testable and maintainable code. Dagger components are typically instantiated within
+  the `ComponentRegistrar` of an SDK, which allows for the injection of dependencies from
+  `firebase-components` into the Dagger graph.
+
+---
+
+## Iteration Loop
+
+After you make a change, here's the flow you should follow:
+
+- Format the code using `spotless`. It can be run with:
+  ```bash
+  ./gradlew :<firebase-project>:spotlessApply
+  ```
+- Run unit tests:
+  ```bash
+  ./gradlew :<firebase-project>:check
+  ```
+- If necessary, run integration tests based on the instructions above.
+
+---
+
+## External Dependencies
+
+---
+
+Do not add, under any circunstance, any new dependency to a SDK that does not already exists in the
+`gradle/libs.versions.toml`, and even then, only do it if cxexplicitly asked to do so. The Firebase
+SDKs are designed to be lightweight, and adding new dependencies can increase the size of the final
+artifacts.
+
+---
+
+## Updating this Guide
+
+If new patterns or conventions are discovered, update this guide to ensure it remains a useful
+resource.