docs: expand README quickstarts and error handling

[however-yir]

Summary

• add a runtime quickstart matrix for Node.js, Bun, Deno, and edge runtimes
• add a root README index for tool use examples and migration notes for older API patterns
• clarify webhook verification setup with a minimal server-side example
• expand common error handling guidance, request ID notes, retry/timeout docs, and add examples/errors.ts

Validation

• npx prettier --write README.md examples/errors.ts
• git diff --check