feat: add multi-device concurrency, server lifecycle CLI, and unified execution stream

- Implement DevicePool and DeviceExecutionLock concurrency modes (per-device FIFO vs global serialization)
- Extend ArtemisClient SDK and CLI with explicit device targeting and concurrency strategy configuration
- Add server lifecycle management (artemis restart, stop, status) with cross-platform PID discovery
- Update MCP server tools to support multi-device dispatch, trace tagging, and per-device execution
- Enhance Showcase UI with step-by-step visual replay, playback speed controls, and device serial badges
- Add comprehensive unit test coverage and edge-case verification scripts

tests/unit/test_prompt_builder.py

@@ -21,7 +21,7 @@ from artemis.agents.operator.prompt_builder import OperatorPromptBuilder
 def test_operator_prompt_builder():
     """Verify prompt rendering formats variables correctly."""
     system_msg = OperatorPromptBuilder.build_system_message()
-    assert "ARTEMIS Operator" in system_msg
+    assert "Android Device Operator" in system_msg
     assert "click" in system_msg
 
     human_msg = OperatorPromptBuilder.build_human_message(