python-sdk: generated type foundation (all v2 schemas) (#13953)

## Summary
Foundation PR only (base for PR #3).

This PR contains the SDK runtime foundation and generated artifacts:

- pinned runtime binary in `sdk/python/bin/` (`codex` or `codex.exe` by
platform)
- single maintenance script:
`sdk/python/scripts/update_sdk_artifacts.py`
- generated protocol/types artifacts under:
  - `sdk/python/src/codex_app_server/generated/protocol_types.py`
  - `sdk/python/src/codex_app_server/generated/schema_types.py`
  - `sdk/python/src/codex_app_server/generated/v2_all/*`
- generation-contract test wiring (`tests/test_contract_generation.py`)

## Release asset behavior
`update_sdk_artifacts.py` now:
- selects latest release by channel (`--channel stable|alpha`)
- resolves the correct asset for current OS/arch
- extracts platform binary (`codex` on macOS/Linux, `codex.exe` on
Windows)
- keeps runtime on single pinned binary source in `sdk/python/bin/`

## Scope boundary
- ✅ PR #2 = binary + generation pipeline + generated types foundation
- ❌ PR #2 does **not** include examples/integration logic polish (that
is PR #3)

## Validation
- Ran: `python scripts/update_sdk_artifacts.py --channel stable`
- Regenerated and committed resulting generated artifacts
- Local tests pass on branch

sdk/python/README.md

@@ -0,0 +1,79 @@
+# Codex App Server Python SDK (Experimental)
+
+Experimental Python SDK for `codex app-server` JSON-RPC v2 over stdio, with a small default surface optimized for real scripts and apps.
+
+The generated wire-model layer is currently sourced from the bundled v2 schema and exposed as Pydantic models with snake_case Python fields that serialize back to the app-server’s camelCase wire format.
+
+## Install
+
+```bash
+cd sdk/python
+python -m pip install -e .
+```
+
+## Quickstart
+
+```python
+from codex_app_server import Codex, TextInput
+
+with Codex() as codex:
+    thread = codex.thread_start(model="gpt-5")
+    result = thread.turn(TextInput("Say hello in one sentence.")).run()
+    print(result.text)
+```
+
+## Docs map
+
+- Golden path tutorial: `docs/getting-started.md`
+- API reference (signatures + behavior): `docs/api-reference.md`
+- Common decisions and pitfalls: `docs/faq.md`
+- Runnable examples index: `examples/README.md`
+- Jupyter walkthrough notebook: `notebooks/sdk_walkthrough.ipynb`
+
+## Examples
+
+Start here:
+
+```bash
+cd sdk/python
+python examples/01_quickstart_constructor/sync.py
+python examples/01_quickstart_constructor/async.py
+```
+
+## Bundled runtime binaries (out of the box)
+
+The SDK ships with platform-specific bundled binaries, so end users do not need updater scripts.
+
+Runtime binary source (single source, no fallback):
+
+- `src/codex_app_server/bin/darwin-arm64/codex`
+- `src/codex_app_server/bin/darwin-x64/codex`
+- `src/codex_app_server/bin/linux-arm64/codex`
+- `src/codex_app_server/bin/linux-x64/codex`
+- `src/codex_app_server/bin/windows-arm64/codex.exe`
+- `src/codex_app_server/bin/windows-x64/codex.exe`
+
+## Maintainer workflow (refresh binaries/types)
+
+```bash
+cd sdk/python
+python scripts/update_sdk_artifacts.py --channel stable --bundle-all-platforms
+# or
+python scripts/update_sdk_artifacts.py --channel alpha --bundle-all-platforms
+```
+
+This refreshes all bundled OS/arch binaries and regenerates protocol-derived Python types.
+
+## Compatibility and versioning
+
+- Package: `codex-app-server-sdk`
+- Current SDK version in this repo: `0.2.0`
+- Python: `>=3.10`
+- Target protocol: Codex `app-server` JSON-RPC v2
+- Recommendation: keep SDK and `codex` CLI reasonably up to date together
+
+## Notes
+
+- `Codex()` is eager and performs startup + `initialize` in the constructor.
+- Use context managers (`with Codex() as codex:`) to ensure shutdown.
+- For transient overload, use `codex_app_server.retry.retry_on_overload`.