test: add corpus golden report snapshots

[ebihara99999]

Purpose

Refs #41. This is stacked on #44 and implements the third PR-plan slice: committed golden reports and an out-of-band regeneration task.

Changes

• Commit one pretty-printed CodeKeeper JSON report per vendored GitLab corpus file under spec/fixtures/golden_reports/gitlab/.
• Generate reports through the existing corpus runner using repository-relative corpus paths, so snapshot paths do not depend on the execution environment.
• Add RSpec coverage that verifies one snapshot exists per corpus file, snapshots match current CodeKeeper output, and every recorded report path is repository-relative.
• Add bundle exec rake corpus:regenerate_golden_reports as the explicit regeneration task.
• Document the snapshot location and regeneration procedure in the corpus README.

Impact

This is test-only. Runtime library behavior is unchanged. The regeneration task is not part of CI; CI only verifies the committed snapshots.

Verification

• bundle exec rake corpus:regenerate_golden_reports
• bundle exec rspec - 80 examples, 0 failures
• bundle exec rubocop - 41 files inspected, no offenses
• git diff --check - no whitespace errors

Scope Out

• Full-repository sweeps
• Additional corpus selection
• RuboCop differential changes beyond the stack base