STATE-WP-0069 T06: add workplan flow entity and prefer open_workplans

Introduce flows/workplan.yaml and /flows/workplan/* routes alongside the
legacy workstream flow. Dashboard, README, and MCP flow docs now prefer
open_workplans with open_workstreams fallback until dual-key retirement.

api/routers/flows.py

@@ -24,12 +24,19 @@ from api.workplan_status import normalize_workplan_status
 
 router = APIRouter(prefix="/flows", tags=["flows"])
 
+_WORKPLAN_FLOW_ENTITY_TYPES = frozenset({"workplan", "workstream"})
+
+
+def _is_workplan_flow(entity_type: str) -> bool:
+    return entity_type in _WORKPLAN_FLOW_ENTITY_TYPES
+
 
 @router.get("/definitions")
 async def list_flow_definitions() -> list[dict[str, Any]]:
     flows = [
         load_flow(entity_type)
         for entity_type in (
+            "workplan",
             "workstream",
             "task",
             "contribution",
@@ -93,7 +100,7 @@ async def advance_workstation(
         )
 
     entity = await _entity(entity_type, entity_id, session)
-    if entity_type == "workstream":
+    if _is_workplan_flow(entity_type):
         transition_workplan_status(entity, target_workstation)
     elif entity_type == "task":
         parent = await session.get(Workplan, entity.workplan_id)
@@ -117,7 +124,9 @@ async def _flow_object(
 ) -> dict[str, Any]:
     entity = await _entity(entity_type, entity_id, session)
     status = _value(entity.status)
-    current_status = normalize_workplan_status(status) if entity_type == "workstream" else status
+    current_status = (
+        normalize_workplan_status(status) if _is_workplan_flow(entity_type) else status
+    )
     obj: dict[str, Any] = {
         "id": str(entity.id),
         "status": current_status,
@@ -125,7 +134,7 @@ async def _flow_object(
         "previous_workstation": current_status,
     }
 
-    if entity_type == "workstream":
+    if _is_workplan_flow(entity_type):
         tasks = list((await session.execute(
             select(Task).where(Task.workplan_id == entity_id)
         )).scalars().all())
@@ -163,6 +172,7 @@ async def _entity(
     session: AsyncSession,
 ):
     model_by_type = {
+        "workplan": Workplan,
         "workstream": Workplan,
         "task": Task,
         "contribution": Contribution,