Add AI SDK relay message-array injection and example app

[khaliqgant]

Summary

• extend the AI SDK communicate adapter so message-array call sites get synthetic relay system injection in addition to string system prompts
• add a compact end-to-end Next.js helpdesk example that uses AI SDK + Relay communicate and escalates to a Relay workflow
• update AI SDK and workflow docs to show messages-based usage and point to the example

Testing

• npm run check (packages/sdk)
• npx tsx --test packages/sdk/src/__tests__/communicate/adapters/ai-sdk.test.ts

Notes

• the new middleware behavior preserves existing system injection while also prepending a synthetic system message for messages arrays
• example app is intentionally small and illustrative rather than production-complete

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