fix(heartbeat): route unified sessions to last channel

tests/cli/test_commands.py

@@ -1787,6 +1787,42 @@ def test_heartbeat_target_skips_archived_webui_sessions():
     assert target == ("websocket", "active")
 
 
+def test_heartbeat_target_uses_last_channel_for_unified_session():
+    from nanobot.cli.commands import _pick_heartbeat_target_from_sessions
+    from nanobot.session.keys import LAST_CHANNEL_METADATA_KEY, UNIFIED_SESSION_KEY
+
+    target = _pick_heartbeat_target_from_sessions(
+        enabled_channels=["telegram", "discord"],
+        archived_keys=[],
+        sessions=[{"key": UNIFIED_SESSION_KEY}],
+        unified_session_metadata={LAST_CHANNEL_METADATA_KEY: "discord:chat-42"},
+    )
+
+    assert target == ("discord", "chat-42")
+
+
+@pytest.mark.parametrize(
+    "metadata",
+    [
+        {"last_channel": "telegram:chat-42"},
+        {"last_channel": "cli:direct"},
+        {"last_channel": "invalid"},
+    ],
+)
+def test_heartbeat_target_rejects_unroutable_unified_metadata(metadata):
+    from nanobot.cli.commands import _pick_heartbeat_target_from_sessions
+    from nanobot.session.keys import UNIFIED_SESSION_KEY
+
+    target = _pick_heartbeat_target_from_sessions(
+        enabled_channels=["discord"],
+        archived_keys=[],
+        sessions=[{"key": UNIFIED_SESSION_KEY}],
+        unified_session_metadata=metadata,
+    )
+
+    assert target == ("cli", "direct")
+
+
 def _write_instance_config(tmp_path: Path) -> Path:
     config_file = tmp_path / "instance" / "config.json"
     config_file.parent.mkdir(parents=True)