feat(terminology): align completion events to State Hub subjects (CUST-WP-0055 T03)

Add org.statehub.workplan.completed as the preferred catalog type, document
org.statehub.workstream.completed as legacy, and deprecate the custodian-era
org.workstream.completed type with a 2026-12-31 sunset date.

tests/test_workplan_event_types.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from activity_core.event_type_registry import parse_event_type_file
+
+_EVENT_DIR = Path(__file__).parent.parent / "event-types"
+
+
+def test_preferred_workplan_completion_event_is_active() -> None:
+    event_type = parse_event_type_file(_EVENT_DIR / "org.statehub.workplan.completed.md")
+
+    assert event_type.type_id == "org.statehub.workplan.completed"
+    assert event_type.status == "active"
+    assert event_type.publisher == "state-hub"
+    assert event_type.attribute_schema["workplan_id"]["required"] is True
+    assert "preferred" in event_type.raw_md.lower()
+
+
+def test_legacy_statehub_completion_event_points_to_replacement() -> None:
+    event_type = parse_event_type_file(_EVENT_DIR / "org.statehub.workstream.completed.md")
+
+    assert event_type.type_id == "org.statehub.workstream.completed"
+    assert event_type.status == "legacy"
+    assert "org.statehub.workplan.completed" in event_type.raw_md
+
+
+def test_custodian_catalog_completion_event_is_deprecated() -> None:
+    event_type = parse_event_type_file(_EVENT_DIR / "org.workstream.completed.md")
+
+    assert event_type.type_id == "org.workstream.completed"
+    assert event_type.status == "deprecated"
+    assert "org.statehub.workplan.completed" in event_type.raw_md
+    assert "2026-12-31" in event_type.raw_md