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 test — 67 runs, 197 assertions, 0 failures, 0 errors, 1 skip.
• bundle exec rubocop — 23 files inspected, no offenses.
• ruby -c exe/railstart — Syntax OK.
• bundle exec rubocop --lint lib/ test/ — 18 files inspected, no offenses.
• gem build railstart.gemspec --output /tmp/railstart-gemfile-lock-cleanup-20260726.gem — succeeded.
• gem specification /tmp/railstart-gemfile-lock-cleanup-20260726.gem files --yaml — Gemfile.lock included: False (19 packaged files).
• git diff --check — passed.