v0.3.0 — transport preflight

worker_preflight.sh — tells you whether it is the network or the credential, before the worker runs.

A sandboxed worker that cannot resolve DNS and one with a bad token fail in ways that read almost identically once a tool re-words the error. An unattended agent then reports the wrong cause and stops.

Output	Exit	Meaning
NET_DISABLED	3	DNS or transport failed. Not a credential problem.
API_UNAVAILABLE	4	Host reachable, endpoint is not. A proxy allowlist missing this host looks exactly like this.
NET_OK	0	Transport proven.
AUTH_OK	0	Transport proven, then credentials accepted.
AUTH_FAILED	5	Transport proven, so the credential conclusion is finally safe.

Two rules:

1. Transport probes run with credential variables cleared in the child process only. Re-injecting a known-good token before proving reachability neither distinguishes auth from DNS nor limits who can read the token.
2. The auth probe runs only after transport is proven, and only when asked.

Nothing prints an environment value.

Suggested by @ooocooc in openai/codex#42402.