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/examples/README.md

@@ -23,11 +23,11 @@ python -m pip install -e .
 
 When running examples from this repo checkout, the SDK source uses the local
 tree and does not bundle a runtime binary. The helper in `examples/_bootstrap.py`
-uses the installed `codex-cli-bin` runtime package.
+uses the installed `openai-codex-cli-bin` runtime package.
 
-If the pinned `codex-cli-bin` runtime is not already installed, the bootstrap
+If the pinned `openai-codex-cli-bin` runtime is not already installed, the bootstrap
 will download the matching GitHub release artifact, stage a temporary local
-`codex-cli-bin` package, install it into your active interpreter, and clean up
+`openai-codex-cli-bin` package, install it into your active interpreter, and clean up
 the temporary files afterward.
 
 Current pinned runtime version: `0.116.0-alpha.1`
@@ -43,7 +43,7 @@ python examples/<example-folder>/async.py
 
 The examples bootstrap local imports from `sdk/python/src` automatically, so no
 SDK wheel install is required. You only need the Python dependencies for your
-active interpreter and an installed `codex-cli-bin` runtime package (either
+active interpreter and an installed `openai-codex-cli-bin` runtime package (either
 already present or automatically provisioned by the bootstrap).
 
 ## Recommended first run