chore: upgrade Ruby 3.4, RSpec 3.13, add RuboCop; remove ActiveSupport and Guard

[adrian-afergon]

Summary

• Upgrade runtime baseline to Ruby 3.4.4 and pin modern toolchain gems
• Upgrade RSpec to 3.13 and add RuboCop + rubocop-rspec for linting
• Remove Guard + terminal-notifier-guard and remove ActiveSupport-based autoloading
• Simplify boot/loading setup and align test helper/config files
• Fix README repository link and refresh documented stack versions

Notes

• Gemfile.lock was intentionally removed and will be regenerated on first �undle install.
• size:exception applies to this modernization series because lockfile churn can exceed review budget.

Validation

• Per constraints, no �undle install or Ruby commands were executed in this change.