feat(investigation): preserve cumulative reference graphs

Accumulate investigation paths and preserve failed attempts across rounds. Keep graph annotations separate from execution state, synchronize the API contract, and consolidate documentation.

tests/unit/test_operational_tasks.py

@@ -314,12 +314,13 @@ async def test_progress_can_publish_without_conclusion_and_replaces_the_complete
     updated = await service.publish_artifacts(task["task_id"], replacement)
     assert updated.progress_revision == 2
     assert [node.node_id for node in updated.task.latest_progress.progress.diagram.node_refs] == ["other"]
-    cleared = await service.publish_artifacts(task["task_id"], AgentArtifactsCommand(
+    retained = await service.publish_artifacts(task["task_id"], AgentArtifactsCommand(
         request_id=uuid4(), source_session_id="session-1", run_id=repository.run["run_id"],
-        progress={"summary": "当前不需要图示。", "diagram": None},
+        progress={"summary": "补充本轮说明。", "diagram": None},
     ))
-    assert cleared.progress_revision == 3
-    assert cleared.task.latest_progress.progress.diagram is None
+    assert retained.progress_revision == 3
+    assert retained.task.latest_progress.progress.diagram.mermaid == updated.task.latest_progress.progress.diagram.mermaid
+    assert retained.task.latest_progress.progress.diagram.edges == []
     assert not repository.notifications
 
 
@@ -1318,7 +1319,7 @@ async def test_final_report_archives_its_progress_diagram_across_new_rounds():
     )
     await service.publish_artifacts(task["task_id"], command)
     assert repository.reports[-1]["report"]["diagram"]["title"] == "调查过程"
-    assert repository.reports[-1]["report"]["diagram"]["node_refs"][0]["status"] == "running"
+    assert repository.reports[-1]["report"]["diagram"]["node_refs"][0]["status"] == "reference"
     assert command.report.diagram is None  # Do not change the idempotency request.
     repository.task["execution_generation"] = 2
     detail = await service.detail(task["task_id"])