### Motivation
- Avoid placing PATH entries under the system temp directory by creating
the helper directory under `CODEX_HOME` instead of
`std::env::temp_dir()`.
- Fail fast on unsafe configuration by rejecting `CODEX_HOME` values
that live under the system temp root to prevent writable PATH entries.
### Testing
- Ran `just fmt`, which completed with a non-blocking
`imports_granularity` warning.
- Ran `just fix -p codex-arg0` (Clippy fixes) which completed
successfully.
- Ran `cargo test -p codex-arg0` and the test run completed
successfully.
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