chore(release): prepare v0.5.1

[dpaluy]

Summary

• Bump Railstart from 0.5.0 to 0.5.1.
• Record the merged actions/checkout v7 maintenance update in the changelog.
• Refresh Gemfile.lock to the new gem version.

Verification

• bundle exec rake test — 67 runs, 197 assertions, 0 failures, 0 errors, 1 skip.
• bundle exec rubocop — 23 files inspected, no offenses.
• bundle exec rake yard — completed successfully.
• gem build railstart.gemspec — built railstart-0.5.1.gem.
• 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.

The release PR is intentionally not merged or published.

[dpaluy]

Closing this duplicate release PR because it is superseded by the verified canonical release PR #38: #38

Evidence as of the #38 head 38cce0fc5c2357dc222e4e009474b844d345db5c:

• Base is current master at 91bae4c5728e2250f9695e5270264e9cf0bb2221.
• The PR is OPEN, MERGEABLE, and CLEAN.
• Ruby 3.4, Ruby 4.0, and Ruby 4.0.5 checks are all green.
• Its release changelog consolidates the merged actions/checkout v7, YARD 0.9.45, and RDoc 8.0.0 updates, with the remaining diff limited to the 0.5.1 version/changelog release changes.

No merge or release action is being performed here.