#198 Apps: Implement App base class and @action decorator

## Parent Epic: #195 ## Description Create the App base class and @action decorator for defining apps. ## Location engine/apps/sdk/base.py ## App Base Class ### Required Class Attributes - name: str (unique identifier) - version: str (semver) ### Optional Class Attributes - display_name: str - description: str - category: str (communication, storage, integration, utility) ### Class Methods - get_manifest() -> dict - Generates JSON manifest from class definition - Includes all actions with parameters/returns schemas ### Instance Methods - execute_action(ctx: AppContext, action_name: str, **kwargs) -> Any - Dispatches to action method - Validates action exists ## @action Decorator ### Parameters - description: str - parameters: dict (JSON Schema) - returns: dict (JSON Schema) - waits_for_event: bool = False ### Behavior - Attaches ActionDefinition to method - Used by get_manifest() to generate schema ## ActionDefinition Dataclass - name, description - parameters, returns (JSON Schema) - waits_for_event ## @app Class Decorator - Auto-collects all @action methods - Validates required attributes exist ## Example Usage ```python @app class MyApp(App): name = 'my_app' version = '1.0.0' @action( description='Do something', parameters={'type': 'object', 'properties': {...}}, returns={'type': 'object'} ) def my_action(self, ctx: AppContext, param1: str) -> dict: return {'result': param1} ``` ## Testing - Test manifest generation - Test action execution - Test validation errors