feat: add generated ReplyNodes Ruby SDK scaffold

[replynodes-ai]

Summary

• initialize the public replynodes/replynodes-ruby repository
• vendor the canonical ReplyNodes OpenAPI contract
• add pinned OpenAPI Generator 7.10.0 Ruby configuration and reproducible generation scripts
• add generated Ruby client plus thin ReplyNodes::Client, response/error, operation registry, README, and local test scaffold
• link source-of-truth issue: replynodes/replynodes-fetcher#297

Verification

• canonical OpenAPI SHA-256: 46e5807e5164904e31dbdc7a14a61f87ffbe0a4617c076066c8685de8d5e6edf
• generated files committed at 596aab5
• git diff --check: passed
• Ruby/Bundler/gem gates were not run on the host because Ruby, Bundler, and gem are not installed.
• The implementation worker was stopped at the operator's request before completing local Ruby tests, generation drift verification, clean gem install, independent review, and final acceptance.
• No live API, public RubyGems publication, release/tag, trusted-publishing, or credential-backed verification is claimed.

Status

This is an intentionally incomplete draft checkpoint. Continue on the same branch/PR; do not create a duplicate PR.

Fallback note: Claude Code was attempted first but its implementation session was blocked by hard filesystem/network sandbox restrictions; GPT-5.6 Luna was used as the authorized fallback.