Title: Improve rollout session initialization error messages
Issue: https://github.com/openai/codex/issues/7283
What: add targeted mapping for rollout/session initialization errors so
users get actionable messages when Codex cannot access session files.
Why: session creation previously returned a generic internal error,
hiding permissions/FS issues and making support harder.
How:
- Added rollout::error::map_session_init_error to translate the more
common io::Error kinds into user-facing hints (permission, missing dir,
file blocking, corruption). Others are passed through directly with
`CodexErr::Fatal`.
- Reused the mapper in Codex session creation to preserve root causes
instead of returning InternalAgentDied.
jif-oai