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_core_config_keys.py

@@ -97,14 +97,8 @@ def _reconcile(tmp_path, config, installed=(), secrets=None):
         _install(plugins_dir, pid)
     secrets_path = tmp_path / "config_secrets.json"
     secrets_path.write_text(json.dumps(secrets or {}), encoding="utf-8")
-    state_manager = Mock()
-    state_manager.get_all_states.return_value = {}
-    plugin_manager = Mock()
-    plugin_manager.plugin_manifests = {}
     reconciler = StateReconciliation(
-        state_manager=state_manager,
         config_manager=_ConfigManager(config, str(secrets_path)),
-        plugin_manager=plugin_manager,
         plugins_dir=plugins_dir,
     )
     return reconciler, reconciler.reconcile_state()