feat(memory): provider actions extension point

Providers with behavior beyond fields (validate a server, start a local
instance, link a CLI profile) previously had no mount besides forking the
panel. Let the schema declare actions and one generic endpoint run them:
ProviderAction on ProviderConfigSchema, POST
/api/memory/providers/{name}/actions/{action} dispatching to
ACTION_HANDLERS in the plugin's config_actions.py (path-loaded and
import-light, like the schema), profile-scoped and off the event loop.
Handlers get the submitted values dict, return a JSON-able result, and
raise ValueError for user-facing 400s. The panel renders declared actions
as generic buttons beside Save. No bundled provider declares actions yet.

tests/plugins/memory/test_config_schema.py

@@ -65,3 +65,9 @@ def test_broken_schema_is_not_cached(monkeypatch, tmp_path):
     recovered = get_provider_config_schema("broken")
     assert recovered is not None
     assert recovered.label == "Broken"
+
+
+def test_provider_without_action_module_has_no_handlers():
+    from plugins.memory.config_schema import get_provider_action_handler
+
+    assert get_provider_action_handler("honcho", "anything") is None