[codex] Clarify project licensing

[wisdom518]

Summary

• add the repository MIT license text at the root
• document contributor licensing expectations in CONTRIBUTING.md
• add MIT license metadata to the frontend package and contract crate

Testing

• git diff --check

Notes

• README already included a basic license section; this PR fills in the missing license file and package metadata.
• scoped to issue Clarify Project Licensing #18 only