Add comprehensive test suite and CI GitHub Action

[nikfot]

Summary

• Add 44 pytest tests guardrailing all existing behavior: Timeseries, Parser, PlotMetrics, check_vmstat_columns, and CLI commands (visualize, compare)
• Add 6 fixture files covering active (vmstat -a), standard (vmstat), headerless, empty, and header-only vmstat log formats
• Add GitHub Actions CI workflow (.github/workflows/ci.yml) that runs the full test suite on push/PR to main using Poetry
• Add pytest dev dependency group and configuration to pyproject.toml
• All vmstat subprocess calls are mocked so tests run without system vmstat

Test Coverage

Module	Tests	What's Covered
test_timeseries.py	8	Init, add_data_point, commit_raw_data mapping, repr
test_parser.py	14	Parse (example, active, no-header, edge cases), PlotMetrics
test_check.py	5	check_vmstat_columns with mocked subprocess (success, errors)
test_plot.py	6	Plot output (5 PNG files, SVG, comparison, invalid metric)
test_cli.py	11	CLI help, visualize, compare, column filter, missing file

Test plan

• All 44 tests pass locally with PYTHONPATH=src pytest -v
• CI workflow runs on this PR