Add wishlist contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new wishlist simple contract example.
• Let each address manage its own wishlist items with sequential per-user item IDs.
• Support item creation, updates, purchased-state changes, deletion, per-user item counts, and paginated item listing.
• Include owner-managed item limits, ownership transfer, input validation, README documentation, schema generation, and root README linking.
• Include six focused unit tests.

Validation

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