Add TypeScript Vercel schema-selection example

[rlippmann]

Summary

• add a small TypeScript schema-selection example under typescript/examples/schema_selection/vercel_ai_sdk_generate_object
• keep schema selection as the enforcement point and treat Vercel AI SDK generateObject as downstream host behavior
• use published @rlippmann/context-compiler with deterministic tests and no directive-drafter dependency

Preserved behavior

• compiler state selects the host-offered schema
• selected schema becomes downstream request config
• no authorized schema means the host omits schema selection and skips downstream generation
• no model output mutates compiler state

Validation

• npm install
• npm run build
• npm run typecheck
• npm test
• npm run example