Add achievement badges contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new achievement-badges simple contract example.
• Let the owner define badge metadata and manage approved issuer accounts.
• Let the owner or approved issuers award badges to holder addresses with one award per holder per badge.
• Support badge archiving, award revocation, ownership transfer, issuer checks, holder badge queries, and badge ownership checks.
• Include README documentation, schema generation, root README linking, and six focused unit tests.

Validation

• cargo fmt --manifest-path achievement-badges/Cargo.toml -- --check
• cargo test --manifest-path achievement-badges/Cargo.toml --target-dir /private/tmp/codex-achievement-badges-target
• git diff --check