feat: add optional AgentPond tracing

[marcusschiesser]

Summary

• add opt-in AgentPond/OpenInference registration through Next.js instrumentation
• enable telemetry on all three Vercel AI SDK generation calls
• document setup using npx agentpond@0.6.0 init --platform vercel

Tracing is disabled unless AGENTPOND_ENABLED=true and exports directly to a connected private Vercel Blob store.

Validation

• Bun lockfile resolution completed successfully
• focused TypeScript check found no errors in the AgentPond instrumentation or telemetry options
• git diff --check

The partial dependency install reports the existing unresolved next/server type used by the API route.

Deployment prerequisites

• private Vercel Blob store
• Vercel System Environment Variables enabled
• AGENTPOND_ENABLED=true

Telemetry content and opt-in

AgentPond remains inactive unless AGENTPOND_ENABLED=true. When enabled, this PR explicitly sets recordInputs: true and recordOutputs: true on every instrumented AI SDK call. This records input and output values that may include prompts, messages, tool data, and generated content. A private Blob store restricts access but does not redact those values. See the Vercel AI SDK telemetry documentation before enabling tracing.