refactor(plugins): no plugin catalog; plugins ship in the image or are mounted (#5374)

First-party plugins have shipped inside the image since a7a281c, and the
catalog's only repository (Steward-plugins) answers 404, so Steward polled a
dead URL every minute for nothing. The operator chose to remove the catalog
entirely (#5214 "Review: readable and fast on the live instance").

Removed:
- core/plugin_index.py and the plugin_catalog_refresh startup task;
- the plugins.index_url default, and migration 0035 deletes the stored
  plugins.index_url / plugins.repositories rows;
- the Settings -> Plugins repository editor, catalog, refresh, install and
  reload routes and their templates;
- the download/checksum/extract install path in plugin_manager, and the
  PLUGIN_INSTALL_DIR config it wrote to;
- plugins/index.yaml, docs/plugins/index.yaml.example and the guide's
  "Publishing to the Catalog" section (now "Shipping a Plugin").

Kept: enabling and configuring plugins, hot-reload on enable, restart, orphan
cleanup, and the external root (/data/plugins) for operator-mounted plugins.
tests/test_no_plugin_catalog.py guards the routes, default and task.

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

steward/config.py

@@ -55,7 +55,7 @@ def load_bootstrap(config_path: Path | str | None = None) -> dict[str, Any]:
     #   • bundled  — first-party plugins shipped inside the image at repo-root
     #     `plugins/`; they version atomically with core and are read-only at runtime.
     #   • external — operator-mounted dir for third-party plugins, persisted in the
-    #     /data volume. Downloads/installs land here, never in the bundled dir.
+    #     /data volume. Steward never downloads plugins; the operator puts them here.
     # Bundled is scanned first, so on a name collision the first-party plugin wins.
     bundled_plugin_dir = raw.get("plugin_dir", "plugins")
     external_plugin_dir = (
@@ -72,8 +72,6 @@ def load_bootstrap(config_path: Path | str | None = None) -> dict[str, Any]:
         "secret_key": secret_key,
         "db_connect_timeout": db_connect_timeout,
         "plugin_dirs": plugin_dirs,
-        # Installs/downloads target the external (writable, persistent) dir.
-        "plugin_install_dir": external_plugin_dir or bundled_plugin_dir,
     }