docs: add AGENT.md (#2657)

Signed-off-by: James Rich <2199651+jamesarich@users.noreply.github.com>

AGENT.md

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+# Meshtastic-Android Project
+
+Meshtastic-Android is a native Android mobile application written in Kotlin. It serves as a client for Meshtastic, an open-source, off-grid, decentralized mesh networking project.
+
+## Architecture
+
+This project is a modern Android application that follows the official architecture guidance from Google. It is a reactive, single-activity app that uses the following:
+
+-   **UI:** Built entirely with Jetpack Compose, including Material 3 components and adaptive layouts for different screen sizes.
+-   **State Management:** Unidirectional Data Flow (UDF) is implemented using Kotlin `Coroutines` and `Flow`s. `ViewModel`s act as state holders, exposing UI state as streams of data.
+-   **Dependency Injection:** Hilt is used for dependency injection throughout the app, simplifying the management of dependencies and improving testability.
+-   **Navigation:** Navigation is handled by Jetpack Navigation for Compose, allowing for a declarative and type-safe way to navigate between screens.
+-   **Data:** The data layer is implemented using the repository pattern.
+    -   **Local Data:** Room and DataStore are used for local data persistence.
+    -   **Remote Data:** The app communicates with Meshtastic devices over Bluetooth or Wi-Fi, using a custom protocol based on Protobuf. It can also connect to MQTT servers. The networking logic is encapsulated in the `:network` module.
+-   **Background Processing:** WorkManager is used for deferrable background tasks.
+
+## Modules
+
+The project is organized into the following modules:
+
+-   `app/`: The main Android application.
+-   `network/`: A library module containing the offline-first networking logic for communicating with the Meshtastic http json api for device hardware and firmware information.
+-   `mesh_service_example/`: An example application demonstrating how to use the AIDL interface to interact with mesh service provided by the main application.
+
+## Commands to Build & Test
+
+The app has two product flavors: `fdroid` and `google`, and two build types: `debug` and `release`.
+
+- Build: `./gradlew assemble{Variant}`. For example, `assembleGoogleDebug` or `assembleFdroidRelease`.
+- Fix linting/formatting: `./gradlew spotlessApply`
+- Run linter checks: `./gradlew detekt`
+- Run local unit tests: `./gradlew test`
+- Run instrumented tests: `./gradlew connectedAndroidTest`
+
+### Creating tests
+
+#### Instrumented tests
+
+- Tests for UI features should use `ComposeTestRule`.
+- UI tests are located in `app/src/androidTest/java/`.
+
+#### Local tests
+
+- Unit tests are located in `app/src/test/java/`.
+- Use [kotlinx.coroutines.test](https://developer.android.com/kotlin/coroutines/test) for testing coroutines.
+
+## Continuous integration
+
+- The CI/CD workflows are defined in `.github/workflows/*.yaml`.
+- These workflows run checks for code style, linting, and tests on every pull request.
+
+## Version control and code location
+
+- The project uses git and is hosted on GitHub at https://github.com/meshtastic/Meshtastic-Android.