docs: clarify inbound scope for Goal Runs

[JJasonSun]

Summary

• Clarify that Goal Runs start outbound calls to the request phone.
• Document that the current Developer API has no inbound calling endpoints and direct users to the Dashboard for number purchase and Inbound Goal configuration.
• Align the Goal Runs guide and API Reference wording.

API contract

No backend API change. This PR clarifies maintainer-approved behavior in the existing public contract.

Checklist

• I kept the change focused and public-facing.
• I removed secrets, private URLs, phone numbers, and customer data.
• The OpenAPI change describes behavior supported by the public API.
• Existing tests cover this documentation change; no test updates were required.
• No visual layout changed; screenshots are not required.
• I ran pnpm run validate.