fix(harness): persist evidence artifact

harness/tests/test_session_repository.py

@@ -64,6 +64,7 @@ def test_filesystem_repository_reads_phase_artifacts_and_appends_decisions(tmp_p
     repository.create(_manifest(session_id))
 
     repository.write_artifact(session_id, "sql_final", "SELECT 1")
+    repository.write_artifact(session_id, "evidence", '{"sources": []}')
     decisions = repository.append_decisions(
         session_id,
         [DecisionInput(type="sql_approved", subject="phase:7", detail="SELECT 1")],
@@ -71,7 +72,9 @@ def test_filesystem_repository_reads_phase_artifacts_and_appends_decisions(tmp_p
     snapshot = repository.get(session_id)
 
     assert repository.read_artifact(session_id, "sql_final") == "SELECT 1"
+    assert repository.read_artifact(session_id, "evidence") == '{"sources": []}'
     assert snapshot.artifacts["sql_final"] == "SELECT 1"
+    assert snapshot.artifacts["evidence"] == '{"sources": []}'
     assert [decision.type for decision in snapshot.decisions] == ["sql_approved"]
     assert decisions[0].seq == 1
 

harness/tht/session/filesystem_repository.py

@@ -22,6 +22,7 @@ from tht.session.store import MANIFEST, SessionError
 _ARTIFACT_FILES = {
     "question": "question.md",
     "schema_linking": "schema_linking.json",
+    "evidence": "evidence.json",
     "sql_final": "sql_final.sql",
     "validation_report": "validation_report.md",
     "retrieval_pack": "retrieval_pack.md",