chore: align repo scaffolding with the package standard

[TheGreatAxios]

Closes CL-9394

Summary

• CLA.md and the CLA Assistant workflow (cla.yml, signatures on a cla-signatures branch) added; CONTRIBUTING links the CLA. LICENSE already matched Interchange's text.
• package.json: dropped main/types (exports covers Node and Bun), added keywords and homepage#readme, build clears dist first, test runs bun test src --pass-with-no-tests (it ran the e2e suite too before); key order is oxfmt's sortPackageJson order.
• tsconfig.json and tsconfig.build.json use the shared templates (the build config inherits types: ["bun"]). bunfig.toml removed: it only set [test] root.
• .gitignore replaced with the shared template.
• ci.yml now matches the standard: single Node 24 job without the matrix, bun run check, bun run test:e2e (the live suite skips without OPENAI_RESPONSES_LIVE_URL), then packs the tarball and imports createOpenAIResponsesAdapter and responsesAdapterFactory under Node.
• CONTRIBUTING matches the template and keeps the e2e instructions and "How it works" notes. AGENTS.md drops the Distribution section that restated package.json.
• CONTRIBUTING gets a Releasing section: releases are manual, as in Interchange (npm version, push tags, gh release create --generate-notes, npm publish).

Commits

1. chore: add the CLA
2. build: align package.json and tsconfig with the package standard
3. ci: run check and a Node pack smoke
4. docs: trim CONTRIBUTING and AGENTS.md

Checks

bun run build, typecheck, lint, format:check, test, test:e2e (live suite skipped without OPENAI_RESPONSES_LIVE_URL) and the Node pack-and-import smoke pass locally.