Codex manual helper fails when checksum headers are absent

[rvjosh]

A tested source fix is available in a public fork:

• Compare: main...rvjosh:codex/openai-docs-missing-checksum
• Commit: rvjosh@9bc5f8b

The repository owner has disabled pull requests, so I am filing the change here as the supported outward channel.

Summary

• Accept a missing x-content-sha256 header while continuing to reject malformed headers and body/header mismatches.
• Fall back to a locally computed SHA-256 digest. Headerless endpoints must still GET on each check, but an unchanged body reuses the existing cache and reports a cache hit.
• Expose verificationMode (header-sha256 or local-sha256) in status output.
• Add a discoverable --help interface documenting the existing no-argument and --status-json behavior.

Why

The current Codex manual endpoint can return a valid body without x-content-sha256. The helper currently treats that optional transport metadata as mandatory and aborts before the OpenAI Docs skill can load the manual.

Verification

node --test skills/.curated/openai-docs/scripts/fetch-codex-manual.test.mjs
4 passed

The regression suite covers headerless local verification/cache reuse, the existing header-backed fast path, checksum mismatch rejection, and help output without a network fetch. A live fetch against the current manual endpoint also completes in local-sha256 mode.