chore: align repo scaffolding with the package standard

[TheGreatAxios]

Closes CL-9408

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; key order is oxfmt's. Version 0.1.0 → 0.1.1 (patch: no public API break in this stack).
• tsconfig.json and tsconfig.build.json use the shared templates (drops allowJs, composite and allowImportingTsExtensions).
• .gitignore replaced with the shared template.
• check.yml replaced by the standard ci.yml: single Node 24 job without the matrix or the pinned Bun version, bun run check, then packs the tarball and imports createCodexResponsesAdapter, withCodexContentTypeRepair, exchangeCodexCode and refreshCodexTokens under Node.
• CONTRIBUTING added from the template, with the live e2e instructions and a manual Releasing section. AGENTS.md drops the Distribution section that restated package.json, lists the test files, and states the peer ranges.

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 CODEX_LIVE_ACCESS_TOKEN) and the Node pack-and-import smoke pass locally. npm pack ships no test files.