feat(FN-3810): define room responder policy helper for hybrid room chat

Defines a room responder policy helper in `@fusion/core` with corresponding types, implements supporting logic in the dashboard chat module, and adds comprehensive tests for the hybrid room manager scenario (FN-3810 step 2).

Fusion-Task-Id: FN-3810

packages/dashboard/src/__tests__/chat-manager-room-hybrid.test.ts

@@ -0,0 +1,101 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { ChatManager } from "../chat.js";
+
+const mockChatStore = {
+  listRoomMembers: vi.fn(),
+  createSession: vi.fn(),
+};
+
+const mockAgentStore = {
+  init: vi.fn(),
+  listAgents: vi.fn(),
+};
+
+describe("ChatManager room hybrid responder resolution", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("returns ambient members when there are no mentions", () => {
+    mockChatStore.listRoomMembers.mockReturnValue([
+      { roomId: "room-1", agentId: "agent-a", role: "member", addedAt: "2026-01-01" },
+      { roomId: "room-1", agentId: "agent-b", role: "member", addedAt: "2026-01-01" },
+    ]);
+
+    const manager = new ChatManager(mockChatStore as any, "/tmp", mockAgentStore as any);
+    const result = (manager as any).resolveRoomResponders(
+      { id: "chat-1", kind: "room", roomId: "room-1" },
+      [],
+      [
+        { id: "agent-a", name: "A" },
+        { id: "agent-b", name: "B" },
+      ],
+    );
+
+    expect(result.direct).toEqual([]);
+    expect(result.ambient.map((agent: any) => agent.id)).toEqual(["agent-a", "agent-b"]);
+  });
+
+  it("routes mentioned member to direct and others to ambient", () => {
+    mockChatStore.listRoomMembers.mockReturnValue([
+      { roomId: "room-1", agentId: "agent-a", role: "member", addedAt: "2026-01-01" },
+      { roomId: "room-1", agentId: "agent-b", role: "member", addedAt: "2026-01-01" },
+    ]);
+
+    const manager = new ChatManager(mockChatStore as any, "/tmp", mockAgentStore as any);
+    const result = (manager as any).resolveRoomResponders(
+      { id: "chat-1", kind: "room", roomId: "room-1" },
+      [{ agentId: "agent-b", agentName: "B" }],
+      [
+        { id: "agent-a", name: "A" },
+        { id: "agent-b", name: "B" },
+      ],
+    );
+
+    expect(result.direct.map((agent: any) => agent.id)).toEqual(["agent-b"]);
+    expect(result.ambient.map((agent: any) => agent.id)).toEqual(["agent-a"]);
+  });
+
+  it("ignores non-member mentions for direct dispatch", () => {
+    mockChatStore.listRoomMembers.mockReturnValue([
+      { roomId: "room-1", agentId: "agent-a", role: "member", addedAt: "2026-01-01" },
+    ]);
+
+    const manager = new ChatManager(mockChatStore as any, "/tmp", mockAgentStore as any);
+    const result = (manager as any).resolveRoomResponders(
+      { id: "chat-1", kind: "room", roomId: "room-1" },
+      [{ agentId: "agent-z", agentName: "Z" }],
+      [
+        { id: "agent-a", name: "A" },
+        { id: "agent-z", name: "Z" },
+      ],
+    );
+
+    expect(result.direct).toEqual([]);
+    expect(result.nonMemberMentions).toEqual([{ agentId: "agent-z", agentName: "Z" }]);
+    expect(result.ambient.map((agent: any) => agent.id)).toEqual(["agent-a"]);
+  });
+
+  it("dedupes duplicate mentions", () => {
+    mockChatStore.listRoomMembers.mockReturnValue([
+      { roomId: "room-1", agentId: "agent-a", role: "member", addedAt: "2026-01-01" },
+      { roomId: "room-1", agentId: "agent-b", role: "member", addedAt: "2026-01-01" },
+    ]);
+
+    const manager = new ChatManager(mockChatStore as any, "/tmp", mockAgentStore as any);
+    const result = (manager as any).resolveRoomResponders(
+      { id: "chat-1", kind: "room", roomId: "room-1" },
+      [
+        { agentId: "agent-b", agentName: "B" },
+        { agentId: "agent-b", agentName: "B" },
+      ],
+      [
+        { id: "agent-a", name: "A" },
+        { id: "agent-b", name: "B" },
+      ],
+    );
+
+    expect(result.direct.map((agent: any) => agent.id)).toEqual(["agent-b"]);
+    expect(result.ambient.map((agent: any) => agent.id)).toEqual(["agent-a"]);
+  });
+});