Stage publishable Python runtime wheels (#18865)

This is PR 2 of the Python SDK PyPI publishing split. [PR
1](https://github.com/openai/codex/pull/18862) refreshed the generated
SDK bindings; this PR makes the runtime package itself publishable, and
PR 3 will wire the SDK package/version pinning to this runtime package.

## Summary
- Rename the runtime distribution to `openai-codex-cli-bin` while
keeping the import package as `codex_cli_bin`.
- Make the runtime package wheel-only and build `py3-none-<platform>`
wheels instead of interpreter-specific wheels.
- Add `stage-runtime --codex-version` and `--platform-tag` so release
staging can produce the platform wheel matrix from Codex release tags.
- Add focused artifact workflow tests for version normalization,
platform tag injection, and runtime wheel metadata.

## Why Rename
There is already an unofficial PyPI package,
[`codex-bin`](https://pypi.org/project/codex-bin/), distributing OpenAI
Codex binaries. Publishing the official SDK runtime dependency as
`openai-codex-cli-bin` makes the ownership clear, avoids confusing the
SDK-pinned runtime wheel with that unowned wrapper, and keeps the import
package unchanged as `codex_cli_bin`.

## Tests
- `uv run --extra dev pytest
tests/test_artifact_workflow_and_binaries.py` -> 21 passed
- `uv run --extra dev python scripts/update_sdk_artifacts.py
stage-runtime /tmp/codex-python-pr2-rebased/runtime-stage
/tmp/codex-python-pr2-rebased/codex --codex-version
rust-v0.116.0-alpha.1 --platform-tag macosx_11_0_arm64`
- `uv run --with build --extra dev python -m build --wheel
/tmp/codex-python-pr2-rebased/runtime-stage`
- `uv run --with twine --extra dev twine check
/tmp/codex-python-pr2-rebased/runtime-stage/dist/openai_codex_cli_bin-0.116.0a1-py3-none-macosx_11_0_arm64.whl`

## Note
- Full `uv run --extra dev pytest` currently fails because regenerating
from schemas already on `main` adds new DeviceKey Python types. I left
that generated catch-up out of this runtime-only PR.

sdk/python/README.md

@@ -11,7 +11,7 @@ cd sdk/python
 python -m pip install -e .
 ```
 
-Published SDK builds pin an exact `codex-cli-bin` runtime dependency. For local
+Published SDK builds pin an exact `openai-codex-cli-bin` runtime dependency. For local
 repo development, either pass `AppServerConfig(codex_bin=...)` to point at a
 local build explicitly, or use the repo examples/notebook bootstrap which
 installs the pinned runtime package automatically.
@@ -53,7 +53,7 @@ python examples/01_quickstart_constructor/async.py
 
 The repo no longer checks `codex` binaries into `sdk/python`.
 
-Published SDK builds are pinned to an exact `codex-cli-bin` package version,
+Published SDK builds are pinned to an exact `openai-codex-cli-bin` package version,
 and that runtime package carries the platform-specific binary for the target
 wheel.
 
@@ -73,7 +73,7 @@ python scripts/update_sdk_artifacts.py \
   --runtime-version 1.2.3
 python scripts/update_sdk_artifacts.py \
   stage-runtime \
-  /tmp/codex-python-release/codex-cli-bin \
+  /tmp/codex-python-release/openai-codex-cli-bin \
   /path/to/codex \
   --runtime-version 1.2.3
 ```
@@ -81,14 +81,14 @@ python scripts/update_sdk_artifacts.py \
 This supports the CI release flow:
 
 - run `generate-types` before packaging
-- stage `codex-app-server-sdk` once with an exact `codex-cli-bin==...` dependency
-- stage `codex-cli-bin` on each supported platform runner with the same pinned runtime version
-- build and publish `codex-cli-bin` as platform wheels only; do not publish an sdist
+- stage `codex-app-server-sdk` once with an exact `openai-codex-cli-bin==...` dependency
+- stage `openai-codex-cli-bin` on each supported platform runner with the same pinned runtime version
+- build and publish `openai-codex-cli-bin` as platform wheels only; do not publish an sdist
 
 ## Compatibility and versioning
 
 - Package: `codex-app-server-sdk`
-- Runtime package: `codex-cli-bin`
+- Runtime package: `openai-codex-cli-bin`
 - Current SDK version in this repo: `0.2.0`
 - Python: `>=3.10`
 - Target protocol: Codex `app-server` JSON-RPC v2