chore: prepare 0.5.1 release

[dpaluy]

Summary

• Bump railstart from 0.5.0 to 0.5.1.
• Consolidate the merged maintenance updates since 0.5.0 in the release changelog:
  • GitHub Actions actions/checkout v5 → v7
  • YARD 0.9.44 → 0.9.45
  • RDoc 6.17.0 → 8.0.0
• Merge the current master branch into the release branch; the remaining PR diff is limited to the release version and changelog.

Verification

• bundle install — complete; 8 Gemfile dependencies and 38 gems installed.
• bundle exec rake test — 67 runs, 197 assertions, 0 failures, 0 errors, 1 skip.
• bundle exec rubocop — 23 files inspected, no offenses detected.
• bundle exec rake yard — completed successfully; 51.59% documented.
• ruby -c exe/railstart — Syntax OK.
• bundle exec exe/railstart version — Railstart v0.5.1.
• RAILS_CONTRACT=1 bundle exec ruby -Itest test/rails_cli_contract_test.rb — 1 run, 4 assertions, 0 failures, 0 errors, 0 skips.
• gem build railstart.gemspec --output /tmp/railstart-0.5.1.gem — built railstart 0.5.1 successfully.
• Gem specification/unpack inspection — 19 expected release files present in the built package.
• git diff --check — passed.

This PR prepares the release only. It must not be merged, tagged, or published by this maintenance job.