fix(host-agent): thread device manager into task runner observer/screenshot

create_task_runner() built TaskRunner's observer/screenshot_provider by
calling describe_screen(device_id)/take_screenshot(device_id) without
manager=, so both silently fell back to the process-global DEFAULT_MANAGER
singleton instead of the Host Agent's real, device-populated DeviceManager.
DEFAULT_MANAGER never has any device registered, so every task's first step
raised DeviceNotFoundError even though the console (which does pass
manager=) showed the same device as connected. Deterministic on every task,
independent of process count.

Add regression tests confirming both lambdas now resolve devices via the
configured manager; verified each fails with the original DeviceNotFoundError
symptom when the fix is reverted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/device-host-agent/host_agent/execution.py

@@ -14,6 +14,8 @@ from runtime.planner_config import PlannerConfig, load_config as load_planner_co
 from runtime.task import TaskRunner
 from storage.task_metadata import TaskMetadataStore
 from storage.timeline import Timeline
+from tools.describe_screen import describe_screen
+from tools.screenshot import take_screenshot
 from workflow.runner import WorkflowRunner
 from workflow.store import WorkflowStore
 
@@ -39,6 +41,10 @@ def create_execution_factories(
     def create_task_runner() -> TaskRunner:
         return TaskRunner(
             executor=Executor(tools=default_tool_registry(manager=manager)),
+            observer=lambda device_id: describe_screen(device_id, manager=manager),
+            screenshot_provider=lambda device_id: take_screenshot(
+                device_id, manager=manager
+            ),
             metadata_store=metadata_store,
             timeline=timeline,
             planner=_host_agent_planner(resolved_host_agent_config),