config: add open-multi-agent to AI Agent Frameworks collection

[JackChen-me]

What changed

Add open-multi-agent/open-multi-agent to the existing AI Agent Frameworks collection.

Open Multi-Agent is an open-source, TypeScript-native framework for controlled and inspectable multi-agent orchestration. This PR changes only the collection's repository list.

Validation

• The collection contains 18 valid, unique repository IDs, with Open Multi-Agent appearing exactly once.
• The canonical repository is public and not archived.
• git diff --check
• The full verify-collection.mjs scan is currently blocked by 175 pre-existing errors on the unchanged upstream baseline; none refers to Open Multi-Agent.

Disclosure: I maintain Open Multi-Agent.

[vercel]

@JackChen-me is attempting to deploy a commit to the pingcap Team on Vercel.

A member of the Team first needs to authorize it.