feat: queue GitHub comment bounty intents

[santoslgl01-web]

Summary

• Add to extract bounty amounts from GitHub issue comments like or
• Queue detected bounty intents into the linked Bountysource user's shopping cart for later checkout
• Avoid duplicate pending cart items for the same issue and amount
• Add model specs for extraction, queuing, and duplicate prevention

Resolves #25

Testing

• (RubyVM syntax compile for changed Ruby/spec files) ✅
• (standalone parser smoke test) ✅
• could not run locally because this environment has Ruby 2.6.10 while the Gemfile requires Ruby 2.7.1.

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[santoslgl01-web]

Hi! Just checking in — please let me know if any changes are needed. Happy to update the PR. Thanks! 🙂

[santoslgl01-web]

Hi! Friendly follow-up on this PR. The CLA check looks to have been addressed already, and the implementation/tests are ready from my side. If you'd like any changes or a different approach, I'm happy to update it. Thanks!