Turn the script into an installable, tested CLI

[devnsko]

The tool was a single main.py with a few latent bugs. This reworks it into a
proper package while keeping the same command surface.

Packaging and usability:

• Add a combinefiles package with a comf console script, so the README's
  hand-rolled .bat/shell wrappers and PATH instructions are no longer needed.
• Add pyproject.toml (hatchling), ruff and pytest config, and CI across
  Linux/macOS/Windows on Python 3.9 and 3.13.
• python main.py keeps working as a thin shim.

Behaviour fixes:

• .gitignore patterns were matched against absolute paths, so they almost never
  matched. They are now matched against repo-relative POSIX paths, with a
  trailing slash for directories, and are applied by default (--no-gitignore
  opts out; the old -g flag is still accepted).
• requirements.txt was saved as UTF-16, which pip cannot parse. Rewritten UTF-8.
• Combining twice used to pull the previous combined.md back in as input.
  The output file and leftover combined.* files are now always skipped, and
  --list previews exactly what a real run would include.
• Binary files were read as text and corrupted the output; they are now
  detected and skipped, along with dotfiles, oversized files and the usual
  noise directories (node_modules, pycache, .venv, ...).
• Unreadable files and undecodable bytes no longer abort the run.

New options: -a/--all, --hidden, --max-size, --style, -l/--list, -v/--verbose,
-q/--quiet, --version, and -o - to write to stdout. Extensions may be given as
py, .py or *.py. Default output is markdown with fenced, syntax-tagged blocks
whose fences grow when a file contains backticks.

Add 26 tests covering discovery, filtering, output formats and the CLI.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01AQMWgFaixEevUt7QnjZv8b