fix(imagegen): enable native images for Codex OAuth

[DescodeNet]

Summary

• Enable the existing native image-generation surface for official OpenAI Codex
  OAuth endpoints while preserving the openai-codex-responses transport.
• Keep proxy defaults conservative and preserve explicit capability opt-in,
  opt-out, and the global disable flag.
• Contribute the bundled image skill when native generation is available,
  without requiring a separate image API key.

Reproduction

1. Log in to the OpenAI Codex provider with ChatGPT OAuth.
2. Select openai-codex/gpt-6-astra without changing its API or endpoint.
3. Request an image using the built-in image-generation capability.

Before this change, the native gate rejected openai-codex-responses before
checking its endpoint. With a compatible gateway configured, the client
generate_image path could choose that gateway instead of the existing OAuth
session. Without client image credentials, the bundled image skill was also
absent.

The fix stays in the two existing image builtins. It does not change provider
authentication, credential precedence, request transport, image persistence, or
model catalogs. Azure and unrelated API behavior remain unchanged.

Verification

• New regressions were run before the production patch: 7 failed, 17 passed.
• Focused image suites after the patch: 10 files, 150 tests passed.
• Independent original-versus-patched gate probe: original failed the native
  capability assertion; patched passed with the same Codex model.
• bun run check: passed.
• bun run build: passed.
• Isolated CLI smoke: 8/8 checks passed.
• RPC get_state: successful.
• Real CLI mock tool loop with openai-responses: 4/4 checks passed.
• Live OpenAI OAuth smoke through the patched automatic extension path:
  the outgoing native tool requested gpt-image-2.5-sunburst, HTTP 200,
  one generated PNG (1254 x 1254, 1,434,514 bytes), and the real credential file
  was unchanged. The observer only inspected the payload; it did not inject or
  modify the image tool.

Broader validation did not pass completely on this macOS machine:

• Root scripts: 206/207 passed. The unchanged issue-446 packaging test fails
  when reading packed.files after npm pack --json; reproduced in the
  original upstream clone and with direct Node execution.
• Workspace suites: 11/12 passed. Coding-agent passed 10629 tests, with five
  ENOTEMPTY RPC cleanup failures and one 1 ms cold-storage timestamp assertion.
  The unchanged upstream clone reproduced the RPC cleanup failure class.
  The timestamp test passed in that isolated baseline run and remains an
  unisolated full-suite timing failure.

These failures are outside the changed image paths. No tests were removed,
skipped, or weakened.

Independent gate review: approved with no blocking findings. The reviewer ran
135 tests across nine relevant files, non-writing Biome checks, and root
typechecking; independently reproduced the base/candidate gate difference; and
validated the live PNG and automatic-injection receipts.

Scope

Regression coverage includes official endpoints, wrong APIs, malformed and
lookalike URLs, explicit capability overrides, OAuth-only skill discovery,
native/client mutual exclusion, and switching away from native generation.
The two existing no-client-credential/native-capable matrix rows now expect the
bundled guidance to be available.

No dependency, lockfile, release changelog, or generated-file changes.

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Summary by cubic

Enables native image generation for official OpenAI Codex OAuth sessions without requiring a separate image API key. The gate previously rejected the openai-codex-responses transport, so these sessions fell back to the client generate_image tool and never got the bundled image skill.

Bug Fixes

• openai-codex-responses on the exact chatgpt.com host now passes the native image gate without changing the model's transport or base URL.
• The bundled gpt-image-gen skill is contributed whenever native generation is active, even without client image credentials.
• Proxies, lookalikes, malformed URLs, explicit opt-out, and the global disable flag keep their prior behavior.

Verification

• Added gate unit tests and a Codex lifecycle suite; updated arbitration truth-table rows for native-without-credentials.
• No dependency, lockfile, changelog, or generated-file changes.

^{Written for commit bfbf6eb. Summary will update on new commits.}