fix(session): use database session count for has_any_sessions (#351)

The previous implementation used `len(self._entries) > 1` to check if any
sessions had ever been created. This failed for single-platform users because
when sessions reset (via /reset, auto-reset, or gateway restart), the entry
for the same session_key is replaced in _entries, not added. So len(_entries)
stays at 1 for users who only use one platform.

Fix: Query the SQLite database's session count instead. The database preserves
historical session records (marked as ended), so session_count() correctly
returns > 1 for returning users even after resets.

This prevents the agent from reintroducing itself to returning users after
every session reset.

Fixes #351

tests/gateway/test_session.py

@@ -324,3 +324,53 @@ class TestSessionStoreEntriesAttribute:
         store._loaded = True
         assert hasattr(store, "_entries")
         assert not hasattr(store, "_sessions")
+
+
+class TestHasAnySessions:
+    """Tests for has_any_sessions() fix (issue #351)."""
+
+    @pytest.fixture
+    def store_with_mock_db(self, tmp_path):
+        """SessionStore with a mocked database."""
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            s = SessionStore(sessions_dir=tmp_path, config=config)
+        s._loaded = True
+        s._entries = {}
+        s._db = MagicMock()
+        return s
+
+    def test_uses_database_count_when_available(self, store_with_mock_db):
+        """has_any_sessions should use database session_count, not len(_entries)."""
+        store = store_with_mock_db
+        # Simulate single-platform user with only 1 entry in memory
+        store._entries = {"telegram:12345": MagicMock()}
+        # But database has 3 sessions (current + 2 previous resets)
+        store._db.session_count.return_value = 3
+
+        assert store.has_any_sessions() is True
+        store._db.session_count.assert_called_once()
+
+    def test_first_session_ever_returns_false(self, store_with_mock_db):
+        """First session ever should return False (only current session in DB)."""
+        store = store_with_mock_db
+        store._entries = {"telegram:12345": MagicMock()}
+        # Database has exactly 1 session (the current one just created)
+        store._db.session_count.return_value = 1
+
+        assert store.has_any_sessions() is False
+
+    def test_fallback_without_database(self, tmp_path):
+        """Should fall back to len(_entries) when DB is not available."""
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            store = SessionStore(sessions_dir=tmp_path, config=config)
+        store._loaded = True
+        store._db = None
+        store._entries = {"key1": MagicMock(), "key2": MagicMock()}
+
+        # > 1 entries means has sessions
+        assert store.has_any_sessions() is True
+
+        store._entries = {"key1": MagicMock()}
+        assert store.has_any_sessions() is False