## Why
Sandbox wrappers replaced the process launch command and did not carry an
`ExecParams.arg0` override through to the inner process.
## What changed
- Route sandboxed Unix launches with a custom `arg0` through a helper mode that
re-execs the requested program with the override.
- Expose the helper executable to the filesystem sandbox and dispatch its mode
from Codex and exec-server test binaries.
## Testing
Add coverage for the prepared sandbox command and an end-to-end remote process
that verifies both the custom `arg0` and filesystem restrictions.
GitOrigin-RevId: c9f8eef3906d184e670184c2eeed250d5895a9ca