Add spend limit wallet contract example

[alexgduarte]

Summary

• Add an original Spend Limit Wallet example contract for native-token allowances
• Include owner-managed spender limits, block-height period resets, native token sends, config updates, queries, and README examples
• Link the new example from the root README

Validation

• cargo fmt --manifest-path spend-limit-wallet/Cargo.toml -- --check
• cargo test --manifest-path spend-limit-wallet/Cargo.toml --target-dir /private/tmp/codex-spend-limit-wallet-target (8 passed)
• git diff --cached --check

Bounty

Submitted for issue #11 simple-contract tier. This contract is original work by @alexgduarte. Stablecoin payout details can be provided privately if accepted.