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_web_api.py

@@ -68,7 +68,6 @@ def client(mock_config_manager, mock_plugin_catalog):
     api_v3.saved_repositories_manager = MagicMock()
     api_v3.schema_manager = MagicMock()
     api_v3.operation_queue = MagicMock()
-    api_v3.plugin_state_manager = MagicMock()
     api_v3.operation_history = MagicMock()
     api_v3.cache_manager = MagicMock()
     # Readers of what the display publishes (app.py wires real ones).
@@ -88,9 +87,6 @@ def client(mock_config_manager, mock_plugin_catalog):
         'properties': {'enabled': {'type': 'boolean'}}
     }
     
-    # Setup state manager mocks
-    api_v3.plugin_state_manager.get_all_states.return_value = {}
-    
     test_app.register_blueprint(api_v3, url_prefix='/api/v3')
     
     with test_app.test_client() as client:
@@ -664,7 +660,6 @@ class TestPluginsAPI:
         from web_interface.blueprints.api_v3 import api_v3
         api_v3.config_manager = mock_config_manager
         api_v3.plugin_catalog = mock_plugin_catalog
-        api_v3.plugin_state_manager = MagicMock()
         api_v3.operation_history = MagicMock()
         
         # Setup plugin manifests