Fix broken demo, flag swallowing, add tests/CI/LICENSE

[mtisza1]

Summary

Addresses all five items from #1, plus several smaller follow-ups.

Changes

Bug fixes:

• Default awk program: When no program and no -f are given, default to {print} so stdin-only invocations work (e.g. some_cmd | awkplot -p hist). This fixes the broken demo.sh steps 1-4.
• Swallowed flag detection: Flags placed after the awk program (which argparse.REMAINDER silently captures) are now detected and produce a clear error message instead of being silently swallowed.
• Empty awk output: Instead of leaking uplot Ruby backtrace on empty input, awkplot now prints awkplot: awk produced no output and exits 1.
• SIGPIPE guard: signal.SIGPIPE is now gated behind hasattr() for Windows compatibility.

New features:

• --version flag (prints awkplot 0.1.0)
• --uplot-args passthrough for arbitrary uplot options (e.g. --uplot-args "--nbins 30")

Infrastructure:

• Added MIT LICENSE file (was declared in pyproject.toml but missing from tree)
• Added pytest test suite (tests/test_awkplot_cli.py) with 30 tests covering parse_size, build_awk_cmd, build_uplot_cmd, swallowed-flag detection, dry-run integration, and --version
• Updated .gitignore to exclude .pytest_cache/
• Updated README.md with new flags

Note: A GitHub Actions CI workflow file (.github/workflows/ci.yml) was prepared but could not be pushed due to OAuth token scope limitations. The file is available locally and can be added manually. It runs pytest across Python 3.9/3.12/3.13 and a demo.sh smoke test.

Test plan

• All 30 pytest tests pass locally
• --dry-run produces correct output for all flag combinations
• --version prints version correctly
• Swallowed flags produce clear error message
• Empty input produces awkplot: awk produced no output
• No-program invocation defaults to {print}

Closes #1

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