Add --changed-files option to report only touched files

[cesarjr]

Summary

• Add --changed-files to report the coverage status of a given list of files (for example, a branch's changed files) instead of the project-wide below-100% view.
• Listed files are shown even when fully covered, so a dev sees the status of everything they touched.
• The list is read from a file (one repo-relative path per line) or from stdin via -; a leading ./ is stripped so git diff --name-only output joins cleanly.
• Only files present in both the list and the coverage data are shown. Coverage is still reported per whole file (percentage + missed lines), not isolated to the diff's added lines.
• Rename the report heading from "Coverage problems" to "Coverage report" (accurate for both the default and the changed-files view).

How to test

• bundle exec rspec — 24 examples, 0 failures.
• bundle exec rubocop — no offenses.
• File list: printf 'app/javascript/routes.jsx\n' > changed.txt && bundle exec exe/couve spec/fixtures/codeclimate.json report.md --changed-files changed.txt — only routes.jsx appears.
• Stdin: git diff --name-only origin/main...HEAD | bundle exec exe/couve coverage.json report.md --changed-files -.
• No flag: bundle exec exe/couve spec/fixtures/codeclimate.json report.md — unchanged behavior (all below-100% files).

Notes

• The default (no-flag) report keeps its existing filter; the only intentional change to it is the heading text.
• File-level by design: a touched file shows its whole coverage, including pre-existing uncovered lines the dev did not write.

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