Add repository hardening files

[vnovitskyi]

Why do we need this?

As requested by AppSec team, this PR adds repository hardening to make the ofx gem compliant with Pennylane's security standards:

• CODEOWNERS file — requires review from @pennylane-hq/transactions-hub, recommends @pennylane-hq/security
• Pre-commit system — rubocop autocorrect, DO NOT COMMIT check, SSH key passphrase enforcement, trailing whitespace, actionlint
• CI checks — GitHub Actions replacing the existing Travis CI config, with test matrix across Ruby 3.0–3.3+head and a CI Summary job
• RuboCop configuration — aligned with Pennylane standards
• Gemfile — adds rubocop, rubocop-performance, rubocop-rspec to dev dependencies

Notion:
https://www.notion.so/scribetech/ofx-442326442d614106b53c13eaa0610faf
https://www.notion.so/scribetech/Repositories-hardening-1cfc3cfcebe842bc88c3ac4fae4a0506