feat: add sustained scenario labels

[hyeokjun32]

Summary

• Added human-readable scenario identity fields to sustained Orchestrator reports.
• Preserved the existing scenario_mode contract while adding scenario_label, scenario_category, and scenario_description.
• Documented labels for normal, overload, sustained_high_load, and device_local modes.

Validation

• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -m pytest tests/test_multi_workload_sustained.py -q -p no:cacheprovider
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -m pytest tests/test_config.py -q -p no:cacheprovider
• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=src .venv/bin/python -m inferedge_orchestrator run-multi-workload-sustained --config configs/agent_multi_workload_sustained_local.json --output /tmp/orchestrator_scenario_label_test.json --frames 8
• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=src .venv/bin/python -m inferedge_orchestrator run-multi-workload-sustained --config configs/agent_multi_workload_sustained_device_local.json --output /tmp/orchestrator_scenario_label_device_test.json --frames 8
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -m pytest -q -p no:cacheprovider
• git diff --check

Notes

• This is a backward-compatible metadata addition for run registry readability.
• It does not change scheduling behavior, queue policy, or source evidence contracts.