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).

model_manager/worker/worker.py

@@ -21,6 +21,8 @@ Environment Variables:
 - TEMPORAL_NAMESPACE: Temporal namespace (default: model-manager)
 - TEMPORAL_USE_TLS: Enable TLS for Temporal connection (default: false)
 - RUNTIME: Runtime identifier used in queue naming (default: single)
+- SIENTIA_RUNTIME_READY: Internal, set by the worker itself before re-executing after
+  install_runtime. Never set it in the deployment: it skips the runtime install.
 - POD_ID: Kubernetes pod identifier for metrics
 - HTTP_METRICS_PORT: Prometheus metrics server port (default: 9090)
 - HTTP_SDK_METRICS_PORT: Temporal SDK metrics port (default: 9091)
@@ -127,8 +129,6 @@ async def main():
 
     metrics_controller = MetricsController(logger=logger)
 
-    logger.custom_info(f'Installing runtime {runtime}', metadata)
-
     plugin_store_parameters = build_plugin_store_config()
     plugin_store = PluginStore(
         base_url=plugin_store_parameters['base_url'],
@@ -147,11 +147,18 @@ async def main():
     )
 
     if os.environ.get('SIENTIA_RUNTIME_READY') != '1':
+        logger.custom_info(f'Installing runtime {runtime}', metadata)
         await plugin_store.install_runtime(runtime_name=runtime)
         os.environ['SIENTIA_RUNTIME_READY'] = '1'
         logger.custom_info(f'Runtime {runtime} installed, re-executing worker', metadata)
-        # ponytail: fresh interpreter so loaded modules match the runtime on disk (SIENTIAPDE-2214)
+        # ponytail: fresh interpreter so loaded modules match the runtime on disk (SIENTIAPDE-2214).
+        # The install_runtime success metrics (count/lag) are lost to the exec; no dashboard uses them.
         os.execv(sys.executable, [sys.executable, *sys.orig_argv[1:]])  # noqa: S606
+    else:
+        logger.custom_info(
+            f'SIENTIA_RUNTIME_READY=1, runtime {runtime} already installed, skipping install',
+            metadata,
+        )
 
     activities = Activities(
         postgres_config=build_postgres_config(),