refactor(gateway): stop writing JSONL in append_to_transcript / rewrite_transcript

state.db is canonical. JSONL transcripts were a transition fallback;
the fallback was removed in the previous commit. Existing *.jsonl files
on disk are left untouched.

tests/run_agent/test_860_dedup.py

@@ -170,33 +170,7 @@ class TestFlushDeduplication:
 # ---------------------------------------------------------------------------
 
 class TestAppendToTranscriptSkipDb:
-    """Verify skip_db=True writes JSONL but not SQLite."""
-
-    @pytest.fixture()
-    def store(self, tmp_path):
-        from gateway.config import GatewayConfig
-        from gateway.session import SessionStore
-        config = GatewayConfig()
-        with patch("gateway.session.SessionStore._ensure_loaded"):
-            s = SessionStore(sessions_dir=tmp_path, config=config)
-        s._db = None  # no SQLite for these JSONL-focused tests
-        s._loaded = True
-        return s
-
-    def test_skip_db_writes_jsonl_only(self, store, tmp_path):
-        """With skip_db=True, message appears in JSONL but not SQLite."""
-        session_id = "test-skip-db"
-        msg = {"role": "assistant", "content": "hello world"}
-        store.append_to_transcript(session_id, msg, skip_db=True)
-
-        # JSONL should have the message
-        jsonl_path = store.get_transcript_path(session_id)
-        assert jsonl_path.exists()
-        with open(jsonl_path) as f:
-            lines = f.readlines()
-        assert len(lines) == 1
-        parsed = json.loads(lines[0])
-        assert parsed["content"] == "hello world"
+    """Verify skip_db=True skips the SQLite write."""
 
     def test_skip_db_prevents_sqlite_write(self, tmp_path):
         """With skip_db=True and a real DB, message does NOT appear in SQLite."""
@@ -223,14 +197,8 @@ class TestAppendToTranscriptSkipDb:
         rows = db.get_messages(session_id)
         assert len(rows) == 0, f"Expected 0 DB rows with skip_db=True, got {len(rows)}"
 
-        # But JSONL should have it
-        jsonl_path = store.get_transcript_path(session_id)
-        with open(jsonl_path) as f:
-            lines = f.readlines()
-        assert len(lines) == 1
-
-    def test_default_writes_both(self, tmp_path):
-        """Without skip_db, message appears in both JSONL and SQLite."""
+    def test_default_writes_to_sqlite(self, tmp_path):
+        """Without skip_db, message appears in SQLite."""
         from gateway.config import GatewayConfig
         from gateway.session import SessionStore
         from hermes_state import SessionDB
@@ -250,13 +218,7 @@ class TestAppendToTranscriptSkipDb:
         msg = {"role": "user", "content": "test message"}
         store.append_to_transcript(session_id, msg)
 
-        # JSONL should have the message
-        jsonl_path = store.get_transcript_path(session_id)
-        with open(jsonl_path) as f:
-            lines = f.readlines()
-        assert len(lines) == 1
-
-        # SQLite should also have the message
+        # SQLite should have the message
         rows = db.get_messages(session_id)
         assert len(rows) == 1