refactor(plugins): the display publishes plugin runtime state; retire plugin_state.json (#690)

Stage 2 of the web plugin catalog, after #688.

- The display publishes a plugin runtime snapshot (plugin_runtime.py) to
  the shared cache: per plugin loaded, lifecycle state, a short redacted
  error summary, the version it loaded and when, plus published_at /
  stale_after / running. Written on change (throttled to 10 s; the
  RUNNING/ENABLED flip of an ordinary update is not a change) and once a
  minute otherwise; cleanup() publishes running: false.
- The web reads it back and restores loaded / state / error_info in
  /api/v3/plugins/installed (plus loaded_version, loaded_at and
  data.runtime). Only a live snapshot counts; stale, stopped or missing
  answers null and says which.
- data/plugin_state.json is retired: every reader and writer moved to
  config + disk (desired) or the snapshot (observed). Nothing in it was
  non-derivable, so nothing is migrated and an existing file is left
  unread. The web-side PluginStateManager (state_manager.py) is removed;
  the display's plugin_state.PluginStateManager is the only state machine.
- StateReconciliation compares config + disk with the snapshot, reporting
  enabled-but-not-loaded and older-version-loaded as no_action findings.
- Backups list installed manifests with enabled from config.json.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_uninstall_and_reconcile_endpoint.py

@@ -33,7 +33,7 @@ from test._api_v3_test_helpers import mock_plugin_catalog
 
 _API_V3_MOCKED_ATTRS = (
     'config_manager', 'plugin_catalog', 'plugin_store_manager',
-    'plugin_state_manager', 'saved_repositories_manager', 'schema_manager',
+    'saved_repositories_manager', 'schema_manager',
     'operation_queue', 'operation_history', 'cache_manager',
     'health_tracker', 'resource_monitor',
 )
@@ -64,8 +64,6 @@ def _make_client():
     api_v3.plugin_catalog = mock_plugin_catalog()
     api_v3.plugin_catalog.plugins_dir = "/tmp"
     api_v3.plugin_store_manager = MagicMock()
-    api_v3.plugin_state_manager = MagicMock()
-    api_v3.plugin_state_manager.get_all_states.return_value = {}
     api_v3.saved_repositories_manager = MagicMock()
     api_v3.schema_manager = MagicMock()
     api_v3.operation_queue = None  # force the direct (non-queue) path