#197 Apps: Implement AppContext SDK wrapper

## 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