[Bug]: Add Well-Known URI for AgentCard to the JSON/gRPC Schema

### What happened?

Set a global constant for well-known URI `.well-known/agent.json` (Or others See #190 )

This can be imported directly into SDKs

### Relevant log output

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look for the JSON/gRPC schema definitions in the codebase, likely in a schema directory or protobuf files. Find where global constants or URIs are defined. Reference issue #190 for context on well-known URIs. The change involves adding a constant for `.well-known/agent.json` to the schema so SDKs can import it.

索引モデルが issue の本文から書いたものです。