## What changed
Add the `codex-multi-env-copy-file` extension, exposing `multi_env_copy_file` when registered by the host. Copy files between distinct selected environments without loading their contents into model context, and return `bytes_copied` on completion.
Require absolute paths in each environment's native path convention and enforce each endpoint's sandbox policy and permission grants. Stream file contents where supported, with a buffered fallback for older executors. Reject same-environment copies and report I/O error kinds without exposing connection details. Warn that failed writes may leave a truncated or partially written target.
## Testing
Add integration tests for binary transfers in both directions, local and remote executors, path validation, opt-in tool visibility, endpoint permissions and grants, and unavailable or disconnected executors. Verify that copied contents stay outside model context.
GitOrigin-RevId: f5d9e423d105019834e0288828cb68c5711b3a49
Adam Perry @ OpenAI
e7de1f4440