[codex] Implement task search and filtering

[wisdom518]

Summary

• add read-only task query helpers for status, reward, deadline, and text search
• surface all tasks in creation order so the contract can be filtered without a separate index
• document the new query helpers in the API reference

Testing

• git diff --check
• cargo test (blocked by existing test scaffolding issues in src/test.rs around the Stellar asset test client)

Notes

• query helpers cover the issue scope directly: status, reward amount, deadline, and title/description search