Edit Issue - .issue.db

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Description

## Parent Epic: #195

## Description
Create the AppContext class that wraps DurableContext with a simplified, app-developer-friendly interface.

## Location
engine/apps/sdk/context.py

## AppContext Class

### Properties
- metadata: AppMetadata (app_id, app_name, version, action_name, tenant_id, workflow_run_id, task_id)
- tenant_id: str
- workflow_run_id: str
- now: datetime (deterministic)

### Checkpointing
- checkpoint(step_name, func, *args, **kwargs) -> T
  - Wraps ctx.step() with action-scoped step names
  - Returns cached result on replay

### Configuration
- get_config(key, default=None) -> Any
  - Dot notation support: 'api.timeout'
  - Returns tenant-specific config

### Secrets
- get_secret(secret_name) -> str
  - Loads from Vault via tenant installation
  - Never logged

### HTTP Requests
- http_request(url, method, headers, json, timeout, checkpoint_name) -> dict
  - Wraps tools.http.request with circuit breaker
  - Optional checkpoint for idempotency

### Events
- emit_event(event_name, payload) -> None
  - Scoped: app.{app_name}.{event_name}
- wait_for_event(step_name, event_name, timeout_seconds) -> dict
  - Durable wait for external event

### Timing
- sleep(step_name, duration_seconds) -> None
  - Durable sleep via ctx.sleep()

### State
- get_state(key, default=None) -> Any
- set_state(key, value) -> None

### Logging
- log(level, message, **extra)
- debug(), info(), warning(), error() convenience methods

## AppMetadata Dataclass
- app_id, app_name, version
- action_name, tenant_id
- workflow_run_id, task_id

## Testing
- Unit tests for each method
- Mock DurableContext for isolation
- Test checkpoint replay behavior

Due Date (YYYY-MM-DD)

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Edit Issue #197