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/docs/getting-started.md

@@ -0,0 +1,75 @@
+# Getting Started
+
+This is the fastest path from install to a multi-turn thread using the minimal SDK surface.
+
+## 1) Install
+
+From repo root:
+
+```bash
+cd sdk/python
+python -m pip install -e .
+```
+
+Requirements:
+
+- Python `>=3.10`
+- bundled runtime binary for your platform (shipped in package)
+- Local Codex auth/session configured
+
+## 2) Run your first turn
+
+```python
+from codex_app_server import Codex, TextInput
+
+with Codex() as codex:
+    print("Server:", codex.metadata.server_name, codex.metadata.server_version)
+
+    thread = codex.thread_start(model="gpt-5")
+    result = thread.turn(TextInput("Say hello in one sentence.")).run()
+
+    print("Thread:", result.thread_id)
+    print("Turn:", result.turn_id)
+    print("Status:", result.status)
+    print("Text:", result.text)
+```
+
+What happened:
+
+- `Codex()` started and initialized `codex app-server`.
+- `thread_start(...)` created a thread.
+- `turn(...).run()` consumed events until `turn/completed` and returned a `TurnResult`.
+
+## 3) Continue the same thread (multi-turn)
+
+```python
+from codex_app_server import Codex, TextInput
+
+with Codex() as codex:
+    thread = codex.thread_start(model="gpt-5")
+
+    first = thread.turn(TextInput("Summarize Rust ownership in 2 bullets.")).run()
+    second = thread.turn(TextInput("Now explain it to a Python developer.")).run()
+
+    print("first:", first.text)
+    print("second:", second.text)
+```
+
+## 4) Resume an existing thread
+
+```python
+from codex_app_server import Codex, TextInput
+
+THREAD_ID = "thr_123"  # replace with a real id
+
+with Codex() as codex:
+    thread = codex.thread(THREAD_ID)
+    result = thread.turn(TextInput("Continue where we left off.")).run()
+    print(result.text)
+```
+
+## 5) Next stops
+
+- API surface and signatures: `docs/api-reference.md`
+- Common decisions/pitfalls: `docs/faq.md`
+- End-to-end runnable examples: `examples/README.md`