docs(worker): document SIENTIA_RUNTIME_READY, log the skipped install, test install failure (SIENTIAPDE-2214)

From the code-review-python pass on #34:
- README, CLAUDE.md and the module docstring: the worker re-executes
  itself after install_runtime, and SIENTIA_RUNTIME_READY is internal;
  setting it in .env/the chart would skip the install silently.
- "Installing runtime" is now logged only on the pass that installs; the
  re-executed pass logs that the install was skipped.
- The ponytail comment notes that the install success metrics are lost
  to the exec.
- New test: a failed install propagates without setting the flag or
  calling os.execv (locks the install -> flag -> exec order).

tests/worker/test_worker.py

@@ -967,6 +967,49 @@ async def test_main_skips_install_when_runtime_ready(
     mock_activities_class.assert_called_once()
 
 
+@pytest.mark.asyncio
+@patch('model_manager.worker.worker.RUNTIME', 'basic')
+@patch('model_manager.worker.worker.Activities', side_effect=_StopBootError)
+@patch('model_manager.worker.worker.NotificationHandler')
+@patch('model_manager.worker.worker.PluginStore')
+@patch('model_manager.worker.worker.build_plugin_store_config', return_value=_PLUGIN_STORE_CONFIG)
+@patch(
+    'model_manager.worker.worker.build_mongodb_config',
+    return_value={'connection_string': 'mongodb://test', 'database_name': 'db', 'uri': 'u'},
+)
+@patch('model_manager.worker.worker.get_logger')
+@patch('model_manager.worker.worker.start_prometheus_server')
+@patch('model_manager.worker.worker.ensure_runtime_directories')
+async def test_main_install_failure_does_not_reexec(
+    _mock_dirs,
+    _mock_prometheus,
+    _mock_get_logger,
+    _mock_mongo,
+    _mock_ps_config,
+    mock_plugin_store_class,
+    _mock_notification,
+    mock_activities_class,
+    monkeypatch,
+):
+    """A failed install propagates without marking the runtime ready or replacing the process."""
+    from model_manager.worker import worker as worker_module
+
+    monkeypatch.setenv('SIENTIA_RUNTIME_READY', '0')
+    mock_plugin_store_class.return_value.install_runtime = AsyncMock(
+        side_effect=RuntimeError('pip failed')
+    )
+
+    with (
+        patch('model_manager.worker.worker.os.execv') as m_execv,
+        pytest.raises(RuntimeError, match='pip failed'),
+    ):
+        await worker_module.main()
+
+    assert os.environ['SIENTIA_RUNTIME_READY'] == '0'
+    m_execv.assert_not_called()
+    mock_activities_class.assert_not_called()
+
+
 @patch('model_manager.worker.worker.asyncio.run')
 def test_main_entrypoint(mock_asyncio_run):
     """Test the __main__ entrypoint."""