Edit Issue - .issue.db

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Description

## 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

Due Date (YYYY-MM-DD)

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Edit Issue #198