dsh-migrate-codex: Securely Migrating DSH Plugins for Codex Environments

`dsh-migrate-codex` is a security plugin for DeepSeek Harness designed to address the risks of missing files and credential leakage during cross-machine migration of Codex environments. The plugin provides a standard `prepare`, `verify`, and `restore` workflow. It uses a whitelist mechanism to migrate only authorized repositories and automatically excludes sensitive files, such as `.git/config` and private keys, to ensure secure migration. It also supports restoring a single working tree and validates SHA256SUMS before restoration, ensuring data integrity and operational idempotency.