chore: add CI workflow and ruff linting

[mloncarevich]

Summary

• Add GitHub Actions CI workflow (test on Python 3.9-3.12)
• Add ruff linter and formatter configuration
• Add CI badge to README
• Update README features (heatmap no longer "coming soon")

CI Workflow

• test: Runs on multiple Python versions, installs package, tests CLI
• lint: Runs ruff check and format verification

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Note

Low Risk
Primarily adds CI/linting configuration and minor formatting-only changes; low risk since it doesn’t change runtime behavior beyond style/metadata updates.

Overview
Adds a GitHub Actions CI workflow with a test matrix for Python 3.9–3.12 that installs the package and smoke-tests the gitstats CLI, plus a separate ruff lint/format check job.

Introduces Ruff configuration in pyproject.toml (lint rules + formatter settings) and updates README.md with a CI badge and refreshed feature list; code changes in gitstats/cli.py and gitstats/parser.py are formatting/import-order adjustments to satisfy the formatter.

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