Add --fail-on-low-coverage flag to exit non-zero on under-covered files

[cesarjr]

Closes #11

Summary

• Add a boolean --fail-on-low-coverage CLI flag. When set, couve writes the report and then exits 1 if any reported file is below the green threshold (rated 🔴 or 🟡, < 66.66%).
• The report is written before exiting, so the offending files stay visible in the report and in any PR comment built from it.
• The offending files are printed to stderr, e.g. couve: coverage below 66.66% in app/foo.rb, app/bar.rb.
• Extract the 33.33 / 66.66 magic numbers into RED_THRESHOLD / GREEN_THRESHOLD constants (shared by percentage_bar_color and percentage_indicator).
• Add public Couve::Parser#low_coverage? and #low_coverage_files. Since the constructor already narrows the source files to the reported set, these automatically respect --changed-files scope — a green or empty changed-file set passes the build.

How to test

• bundle exec rspec — 35 examples, 0 failures.
• bin/rubocop — no offenses.
• Fail case: couve spec/fixtures/codeclimate.json report.md --fail-on-low-coverage exits 1 (the fixture's people_resolver.rb is at 60%) and prints the offending file to stderr, while still writing report.md.
• Pass case: scope to a green file and confirm exit 0: echo app/javascript/routes.jsx | couve spec/fixtures/codeclimate.json report.md --changed-files - --fail-on-low-coverage.

Notes

• Reuses the existing fixed threshold band; no new or configurable threshold.
• No version bump or CHANGELOG entry in this PR (intentionally deferred to the release).
• Follow-up (separate repo): wire the flag into facil123's .semaphore/semaphore.yml Coverage block so it posts the comment first, then fails.

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