Validate output directory paths

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Summary

• add a shared output directory guard before writing tables, JSON, or plots
• report file-valued output directories with a clear ValueError instead of raw filesystem errors
• cover run and plot CLI failures without tracebacks

Tests

• pytest tests/test_cli_errors.py tests/test_pipeline_e2e.py
• pytest
• python -m telemetry_window_demo.cli run --config configs/default.yaml