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.

tests/test_task_flow_engine.py

@@ -173,9 +173,27 @@ def test_yaml_flow_definitions_load_and_evaluate_representative_entities():
         "capability_request",
         "contribution",
         "task",
+        "workplan",
         "workstream",
     ]
 
+    workplan_result = FlowEngine(
+        custom_ops={
+            "dependencies.any_incomplete": lambda assertion, obj, values: any(
+                value not in assertion.value for value in values
+            )
+        }
+    ).evaluate(
+        {
+            "status": "active",
+            "tasks": [{"status": "done"}],
+            "dependencies": [{"workstation": "finished"}],
+        },
+        flows["workplan"],
+    )
+    assert "finished" in workplan_result.reachable
+    assert "blocked" in [item.workstation for item in workplan_result.unreachable]
+
     workstream_result = FlowEngine(
         custom_ops={
             "dependencies.any_incomplete": lambda assertion, obj, values: any(