fix(gateway): gate session.tool mirrors behind session subscriptions (#124732)

session.tool mirrors the raw agent tool event (full args/result
snapshots) onto session subscribers, but the event name was missing from
SESSION_SUBSCRIPTION_EVENTS. Clients with the session-scoped-events cap
(or browser copilot) that subscribed to one session's messages therefore
received every session's tool payloads through the mirror — the exact
data the registry gate suppresses on the raw agent event. session.observer
was added to the set for precisely this mirror-event class; session.tool
carries strictly more data.

Root cause: the gate is an allowlist and the newer mirror event never
joined it. One set entry; the payload already carries sessionKey so
registry keying works unchanged, and unscoped Control UI clients keep
full fanout.

Regression: new board test drives the server-chat-style targeted fanout
and asserts a scoped client subscribed to a different session receives
nothing — fails pre-fix.

src/gateway/server-broadcast.board.test.ts

@@ -164,6 +164,38 @@ describe("collaboration event scope guards", () => {
     expect(unsubscribed.socket.events).toEqual([]);
   });
 
+  it("suppresses session.tool mirrors for scoped clients without a matching subscription", () => {
+    const subscribed = makeClient("subscribed", "operator", ["operator.read"]);
+    const otherSession = makeClient("other-session", "operator", ["operator.read"]);
+    const unscoped = makeClient("unscoped", "operator", ["operator.read"]);
+    for (const entry of [subscribed, otherSession]) {
+      entry.client.connect.caps = [GATEWAY_CLIENT_CAPS.SESSION_SCOPED_EVENTS];
+    }
+    const sessionMessageSubscribers = createSessionMessageSubscriberRegistry();
+    sessionMessageSubscribers.subscribe(subscribed.client.connId, "agent:main:main");
+    sessionMessageSubscribers.subscribe(otherSession.client.connId, "agent:main:other");
+    const { broadcastToConnIds } = createGatewayBroadcaster({
+      clients: new Set([subscribed.client, otherSession.client, unscoped.client]),
+      sessionMessageSubscribers,
+    });
+
+    // Mirrors the server-chat session.tool fanout: targeted at all session
+    // subscribers, session identity carried in the payload.
+    broadcastToConnIds(
+      "session.tool",
+      { sessionKey: "agent:main:main", tool: { name: "exec", args: { command: "ls" } } },
+      new Set(["subscribed", "other-session", "unscoped"]),
+      { dropIfSlow: true },
+    );
+
+    expect(subscribed.socket.events).toEqual(["session.tool"]);
+    // The scoped client subscribed to a different session must not receive
+    // another session's tool args via the mirror event.
+    expect(otherSession.socket.events).toEqual([]);
+    // Unscoped Control UI clients keep full fanout.
+    expect(unscoped.socket.events).toEqual(["session.tool"]);
+  });
+
   it("delivers prepared global observer audiences exactly once", () => {
     const main = makeClient("main", "operator", ["operator.read"]);
     const legacy = makeClient("legacy", "operator", ["operator.read"]);

src/gateway/server-broadcast.ts

@@ -103,6 +103,10 @@ const SESSION_SUBSCRIPTION_EVENTS = new Set([
   "chat",
   "chat.side_result",
   "session.observer",
+  // Mirrors the raw agent tool event (full args/result snapshots) onto
+  // session subscribers; omitting it here would hand scoped clients the
+  // exact payload the registry gate suppresses on the `agent` event.
+  "session.tool",
 ]);
 
 function serializeFrameField(name: "payload" | "stateVersion", value: unknown): string {