feat: first-meeting gate on drawer Add-guest form (T72.2)

When a host->candidate edge already exists from a prior chat, the
Add-guest form renders the prose textarea disabled with an "already
know each other" note. Submission without the explicit "re-seed
anyway" toggle skips seed_inter_bot_edges so existing edge content
(affinity, trust, knowledge, summaries) survives — guest_added and
group_node_initialized still fire. A small inline script enables /
disables the textarea per-option based on a pre-computed
existing_guest_edges dict surfaced by the GET handler.

tests/test_drawer_guest.py

@@ -265,6 +265,162 @@ def test_drawer_remove_guest_clears_and_closes_scene(client, tmp_path):
         assert kinds.index("scene_closed") < kinds.index("guest_removed")
 
 
+# --- T72.2 first-meeting gate ----------------------------------------------
+
+
+def _seed_host_to_guest_edge(db: Path) -> None:
+    """Materialise a bot_a -> bot_b edge so the gate's check fires."""
+    from chat.eventlog.log import append_and_apply
+
+    with open_db(db) as conn:
+        append_and_apply(
+            conn,
+            kind="edge_update",
+            payload={
+                "source_id": "bot_a",
+                "target_id": "bot_b",
+                "chat_id": "chat_bot_a",
+                "affinity_delta": 0,
+                "knowledge_facts": ["already met before"],
+            },
+        )
+
+
+def test_add_guest_form_disables_prose_when_edge_exists(client, tmp_path):
+    """When host->candidate edge already exists, the GET partial renders
+    the textarea disabled and surfaces the "already know each other"
+    message so the user knows submitting will skip the seed.
+    """
+    _seed_chat(tmp_path / "test.db")
+    _seed_host_to_guest_edge(tmp_path / "test.db")
+
+    response = client.get("/chats/chat_bot_a/drawer")
+    assert response.status_code == 200
+    body = response.text
+    # Note + disabled state both present. The textarea sits next to the
+    # ``add-guest-prose`` class so we can match it specifically.
+    assert "already know each other" in body
+    assert 'class="add-guest-prose"' in body
+    # The textarea for the first (auto-selected) candidate should be
+    # disabled in the initial markup since an edge exists.
+    assert "disabled" in body.split('class="add-guest-prose"', 1)[1].split(">", 1)[0]
+    # And the option carries the ``data-existing-edge="true"`` attribute
+    # the inline JS uses to flip state on subsequent select changes.
+    assert 'data-existing-edge="true"' in body
+
+
+def test_add_guest_with_existing_edge_skips_seed_call(client, tmp_path):
+    """Submitting the Add-guest form WITHOUT toggling re-seed must skip
+    ``seed_inter_bot_edges`` entirely. We assert this via an empty mock
+    queue: if the seed function had been called it would have consumed
+    a canned response (or raised because none was available).
+    """
+    _seed_chat(tmp_path / "test.db")
+    _seed_host_to_guest_edge(tmp_path / "test.db")
+
+    # Empty queue: any classifier call would raise inside MockLLMClient.
+    canned_queue: list[str] = []
+    _override_llm(canned_queue)
+    try:
+        response = client.post(
+            "/chats/chat_bot_a/drawer/guest/add",
+            data={
+                "guest_bot_id": "bot_b",
+                "relationship_prose": "ignored prose",
+                # NO reseed flag — gate should suppress the seed call.
+            },
+        )
+        assert response.status_code == 200
+    finally:
+        app.dependency_overrides.clear()
+
+    with open_db(tmp_path / "test.db") as conn:
+        from chat.state.edges import get_edge
+        from chat.state.world import get_chat
+
+        chat = get_chat(conn, "chat_bot_a")
+        assert chat["guest_bot_id"] == "bot_b"
+
+        # The pre-seeded knowledge fact survives — proof the seed didn't run
+        # and overwrite the existing edge.
+        edge = get_edge(conn, "bot_a", "bot_b")
+        assert "already met before" in edge["knowledge"]
+
+        # Exactly one guest_added; no new edge_update events between
+        # bot_a and bot_b (the pre-seed edge_update from the test setup
+        # is the only edge_update on this pair).
+        added = conn.execute(
+            "SELECT COUNT(*) FROM event_log WHERE kind = 'guest_added'"
+        ).fetchone()[0]
+        assert added == 1
+
+        edge_updates = conn.execute(
+            "SELECT payload_json FROM event_log WHERE kind = 'edge_update'"
+        ).fetchall()
+        # Only the pre-seed edge_update from _seed_host_to_guest_edge.
+        ab_updates = [
+            json.loads(p[0])
+            for p in edge_updates
+            if {
+                json.loads(p[0]).get("source_id"),
+                json.loads(p[0]).get("target_id"),
+            }
+            == {"bot_a", "bot_b"}
+        ]
+        assert len(ab_updates) == 1
+        assert ab_updates[0]["knowledge_facts"] == ["already met before"]
+
+
+def test_add_guest_with_existing_edge_and_reseed_runs_seed(client, tmp_path):
+    """Toggling ``re-seed anyway`` flips the gate off — the existing
+    flow runs (seed produces deltas, two ``edge_update`` events fire).
+    """
+    _seed_chat(tmp_path / "test.db")
+    _seed_host_to_guest_edge(tmp_path / "test.db")
+
+    canned = json.dumps(
+        {
+            "a_to_b_summary": "reconnected",
+            "a_to_b_knowledge_facts": ["new fact"],
+            "a_to_b_affinity_delta": 2,
+            "a_to_b_trust_delta": 1,
+            "b_to_a_summary": "reconnected",
+            "b_to_a_knowledge_facts": [],
+            "b_to_a_affinity_delta": 1,
+            "b_to_a_trust_delta": 0,
+        }
+    )
+    _override_llm([canned])
+    try:
+        response = client.post(
+            "/chats/chat_bot_a/drawer/guest/add",
+            data={
+                "guest_bot_id": "bot_b",
+                "relationship_prose": "fresh prose",
+                "reseed": "1",
+            },
+        )
+        assert response.status_code == 200
+    finally:
+        app.dependency_overrides.clear()
+
+    with open_db(tmp_path / "test.db") as conn:
+        edge_updates = conn.execute(
+            "SELECT payload_json FROM event_log WHERE kind = 'edge_update'"
+        ).fetchall()
+        # Pre-seed (1) + two from the re-seed = 3 edge_updates total.
+        ab_updates = [
+            json.loads(p[0])
+            for p in edge_updates
+            if {
+                json.loads(p[0]).get("source_id"),
+                json.loads(p[0]).get("target_id"),
+            }
+            == {"bot_a", "bot_b"}
+        ]
+        assert len(ab_updates) == 3
+
+
 def test_drawer_with_guest_renders_guest_and_group_sections(client, tmp_path):
     _seed_chat(tmp_path / "test.db")
     from chat.eventlog.log import append_and_apply