feat: enforce agent routing and persist restart state

Task 4: stale room blocking + agent_id binding
- MatrixBot._check_agent_routing: blocks normal messages when user has no
  selected agent or room is bound to a different agent
- agent_routing_enabled flag on MatrixRuntime activates the check only
  in real multi-agent mode (RoutedPlatformClient)
- make_handle_new_chat now writes agent_id into new room metadata when
  user already has a selected agent

Task 5: durable restart state tests
- test_restart_persistence.py proves selected_agent_id, room agent_id,
  platform_chat_id, and the sequence counter all survive SQLiteStore
  close/reopen; also covers clean startup with no prior state

tests/adapter/matrix/test_restart_persistence.py

@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+import pytest
+
+from core.store import SQLiteStore
+from adapter.matrix.store import (
+    PLATFORM_CHAT_SEQ_KEY,
+    get_room_meta,
+    get_selected_agent_id,
+    next_platform_chat_id,
+    set_room_meta,
+    set_selected_agent_id,
+)
+
+
+async def test_selected_agent_id_survives_restart(tmp_path):
+    db = str(tmp_path / "state.db")
+    store = SQLiteStore(db)
+    await set_selected_agent_id(store, "@alice:example.org", "agent-2")
+
+    store2 = SQLiteStore(db)
+    assert await get_selected_agent_id(store2, "@alice:example.org") == "agent-2"
+
+
+async def test_room_agent_id_and_platform_chat_id_survive_restart(tmp_path):
+    db = str(tmp_path / "state.db")
+    store = SQLiteStore(db)
+    await set_room_meta(store, "!room:example.org", {
+        "room_type": "chat",
+        "agent_id": "agent-1",
+        "platform_chat_id": "42",
+    })
+
+    store2 = SQLiteStore(db)
+    meta = await get_room_meta(store2, "!room:example.org")
+    assert meta is not None
+    assert meta["agent_id"] == "agent-1"
+    assert meta["platform_chat_id"] == "42"
+
+
+async def test_platform_chat_seq_survives_restart(tmp_path):
+    db = str(tmp_path / "state.db")
+    store = SQLiteStore(db)
+    assert await next_platform_chat_id(store) == "1"
+    assert await next_platform_chat_id(store) == "2"
+    assert await next_platform_chat_id(store) == "3"
+
+    store2 = SQLiteStore(db)
+    assert await next_platform_chat_id(store2) == "4"
+
+
+async def test_routing_state_survives_restart_and_routes_correctly(tmp_path):
+    db = str(tmp_path / "state.db")
+    store = SQLiteStore(db)
+    await set_selected_agent_id(store, "@bob:example.org", "agent-1")
+    await set_room_meta(store, "!convo:example.org", {
+        "room_type": "chat",
+        "agent_id": "agent-1",
+        "platform_chat_id": "10",
+    })
+
+    store2 = SQLiteStore(db)
+    selected = await get_selected_agent_id(store2, "@bob:example.org")
+    meta = await get_room_meta(store2, "!convo:example.org")
+    assert selected == "agent-1"
+    assert meta is not None
+    assert meta["agent_id"] == selected
+    assert meta["platform_chat_id"] == "10"
+
+
+async def test_missing_durable_store_starts_clean(tmp_path):
+    db = str(tmp_path / "brand_new.db")
+    store = SQLiteStore(db)
+    assert await get_selected_agent_id(store, "@nobody:example.org") is None
+    assert await get_room_meta(store, "!nonexistent:example.org") is None