Add CI matrix, ruff config and fix the lint it found

[saeed205]

CI (.github/workflows/ci.yml) runs the suite on ubuntu / windows / macos against Python 3.9 and 3.12 - the declared floor and a current release - plus a CLI smoke test and a python -m build check. The suite is standard-library only and opens no sockets, so it runs unchanged on a sandboxed runner with no egress.

Ruff config selects E,F,W,I,B,C4. UP (pyupgrade) is deliberately excluded and the reasoning is in a comment in ruff.toml: it wants Dict[str, int] rewritten to dict[str, int] and Optional[X] to X | None, which this codebase avoids on purpose while 3.9 is the supported floor. Turning it on meant 68 findings that were all style disagreements rather than defects.

The genuine findings are fixed here: unsorted imports, a redundant open() mode, an unused Iterator import, and two over-length lines. F401 is scoped to commands.py only, where the unused imports are the registry mechanism rather than an oversight.

Also adds .gitignore - netprobe.ini is ignored while netprobe.ini.example stays tracked as the template.

ruff check . is clean and all 84 tests pass locally.