fix(session): coerce null session metadata to empty dict

tests/session/test_consolidated_offset_clamp.py

@@ -58,3 +58,32 @@ def test_valid_offset_is_preserved():
     session = _session(10, 4)
     assert session.last_consolidated == 4
     assert len(session.get_history()) == 6
+
+
+def test_loaded_null_metadata_becomes_empty_dict(tmp_path: Path):
+    """Session jsonl metadata:null must load as {} so agent .pop/.get work."""
+    manager = SessionManager(tmp_path)
+    path = manager._get_session_path("chan:chat")
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(
+        json.dumps({
+            "_type": "metadata",
+            "key": "chan:chat",
+            "created_at": "2026-01-01T00:00:00",
+            "updated_at": "2026-01-01T00:00:00",
+            "metadata": None,
+            "last_consolidated": 0,
+        }) + "\n",
+        encoding="utf-8",
+    )
+    session = manager.get_or_create("chan:chat")
+    assert session.metadata == {}
+    session.metadata["title"] = "ok"
+    assert session.metadata["title"] == "ok"
+    session.metadata.pop("title", None)
+    assert session.metadata == {}
+
+
+def test_session_post_init_coerces_null_metadata():
+    session = Session(key="chan:chat", metadata=None)  # type: ignore[arg-type]
+    assert session.metadata == {}