feat(graph): namespace topic-tunnel rooms with "topic:" prefix + kind field

Previously a cross-wing topic tunnel for "Angular" stored the room as
"Angular" — colliding with a wing's literal folder-derived "Angular" room
at follow_tunnels/list_tunnels read time, and exposing raw topic strings
(which may contain characters rejected by sanitize_name) to the MCP
surface.

Topic tunnels now store their room as "topic:<original-casing>" and carry
kind="topic" on the stored dict. Explicit tunnels get kind="explicit"
(default). follow_tunnels("wing", "Angular") on a literal Angular room
no longer surfaces topic connections for the same name, and any LLM
scanning list_tunnels has a visible discriminator.

tests/test_miner.py

@@ -536,7 +536,10 @@ def test_mine_creates_topic_tunnels_for_shared_topics(tmp_path, monkeypatch):
     listed = palace_graph.list_tunnels()
     assert len(listed) == 1
     rooms = {listed[0]["source"]["room"], listed[0]["target"]["room"]}
-    assert rooms == {"foo"}
+    # Topic tunnels use a ``topic:<name>`` synthetic room so they can't
+    # collide with literal folder-derived rooms of the same name.
+    assert rooms == {"topic:foo"}
+    assert listed[0]["kind"] == "topic"
     wings = {listed[0]["source"]["wing"], listed[0]["target"]["wing"]}
     assert wings == {"wing_one", "wing_two"}