[verified] fix: emit UTC-offset last_active so the app can sort by recency

Session-store updated_at values are naive local datetimes; passing them
through without an offset broke Instant.parse() on the client, dropping
last_active and leaving the chat list unsorted after restart.

tests/test_pheby.py

@@ -12,6 +12,7 @@ by ``conftest.py``).
 from __future__ import annotations
 
 import asyncio
+import datetime as dt
 import json
 import threading
 import time
@@ -459,6 +460,24 @@ class TestBridge:
         assert history[0]["ts"] == "2026-09-10T00:26:40.250000+00:00"
         assert history[1]["ts"] == "2026-09-10T00:26:41.500000+00:00"
 
+    def test_iso_parses_by_instant_for_naive_local_datetimes(self):
+        # gateway session-store timestamps are naive LOCAL datetimes
+        # (gateway.session_lifecycle._now). last_active must come back with a
+        # UTC offset, or the Kotlin client's Instant.parse() fails and the
+        # chat list loses its recency ordering.
+        from pheby import hermes_bridge as hb
+
+        naive = dt.datetime(2026, 9, 10, 5, 41, 48, 401094)  # naive local
+        got = hb._iso(naive)
+        assert got is not None
+        assert got.endswith("+00:00")
+        # Java-style Instant.parse equivalent: parse with offset required.
+        parsed = dt.datetime.fromisoformat(got)
+        assert parsed.tzinfo is not None
+        # Aware datetimes pass through normalized to UTC unchanged.
+        aware = dt.datetime(2026, 9, 10, 12, 0, tzinfo=dt.timezone(dt.timedelta(hours=-5)))
+        assert hb._iso(aware) == "2026-09-10T17:00:00+00:00"
+
     @pytest.mark.asyncio
     async def test_tool_start_event_is_structured_not_text(self, tmp_path,
                                                             monkeypatch):