chore: remove tracked Gemfile.lock

[dpaluy]

Summary

• Remove the tracked development-only Gemfile.lock.
• Add an anchored root /Gemfile.lock ignore rule.
• Keep the existing gemspec packaging policy unchanged: it already excludes Gemfile*, so the lockfile is not packaged.

Verification

• bundle exec rake — 119 runs, 241 assertions, 0 failures, 0 errors, 0 skips; RuboCop inspected 32 files with no offenses.
• bundle exec rake test — 119 runs, 241 assertions, 0 failures, 0 errors, 0 skips.
• bundle exec rubocop — 32 files inspected, no offenses.
• gem build reducto_ai.gemspec --output /tmp/reducto-ai-gemfile-lock-cleanup-20260726.gem — succeeded.
• gem specification /tmp/reducto-ai-gemfile-lock-cleanup-20260726.gem files --yaml — Gemfile.lock included: False (23 packaged files).
• git diff --check — passed.