Add Open Badges 3.0 example credential (VCDM 2.0)

[PatStLouis]

Summary

Add a playground-ready Open Badges 3.0 example expressed as a VCDM 2.0 VerifiableCredential with type AchievementCredential, using the published IMS context (context-3.0.3.json) alongside https://www.w3.org/ns/credentials/v2.

Motivation

• Demonstrates OB 3.0 AchievementCredential / AchievementSubject / Achievement shapes next to other v2 examples.
• Gives issuers and tooling a concrete sample for eddsa-rdfc-2022 and ecdsa-sd-2023 query flows.

Suggested scope

• New directory under credentials/ with credential.json, queries.json, and image.png.
• Copy and wording suitable for the VC Playground (not production badges).

---

Draft implementation: OpSecId/vc-examples branch feature/open-badges-3-achievement.