Validate demo root paths

[stacknil]

Summary

• validate demo-root paths before entering demo pipelines
• report missing demo roots and file-valued demo roots with clear CLI errors
• cover all three demo commands for file-valued demo-root failures without tracebacks

Tests

• pytest tests/test_cli_errors.py tests/test_ai_assisted_detection_demo.py tests/test_rule_evaluation_and_dedup_demo.py tests/test_config_change_investigation_demo.py
• pytest
• python -m telemetry_window_demo.cli run --config configs/default.yaml
• python -m telemetry_window_demo.cli run-ai-demo
• python -m telemetry_window_demo.cli run-rule-dedup-demo
• python -m telemetry_window_demo.cli run-config-change-demo