Align agent guidance with Navigation 3

The root agent guidance still described Navigation 2 for Compose, but the app now uses the Navigation 3 APIs with NavKey state, NavDisplay, and adaptive scene strategies. Update the architecture note so future agents follow the codebase that is actually present.

Constraint: Documentation-only correction for AGENTS.md

Rejected: Mention Jetpack Navigation 2 compatibility | the implementation imports androidx.navigation3 and renders with NavDisplay

Confidence: high

Scope-risk: narrow

Tested: git diff --check -- AGENTS.md

Tested: rg confirmed androidx.navigation3 imports, NavDisplay usage, and AGENTS.md no longer references Navigation 2 for Compose

Not-tested: Full Gradle build not run because this is a documentation-only change

AGENTS.md

@@ -11,7 +11,7 @@ This project is a modern Android application that follows the official architect
 -   **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 2 for Compose, allowing for a declarative and type-safe way to navigate between screens.
+-   **Navigation:** Navigation is handled by Jetpack Navigation 3 with `NavKey`-based back stacks, `NavDisplay`, and adaptive navigation scenes.
 -   **Data:** The data layer is implemented using the repository pattern.
     -   **Local Data:** Room and DataStore are used for local data persistence.
     -   **Remote Data:** Retrofit and OkHttp are used for fetching data from the network.