feat(FN-3971): add aggregate mailbox view for all agents

Adds an "All Agents" unified mailbox view to the dashboard, including a new aggregate API endpoint in `@fusion/core`, updated MailboxView UI with selection controls, CSS styling, and comprehensive tests across core and dashboard packages, with documentation updates.

Fusion-Task-Id: FN-3971

packages/core/src/__tests__/message-store.test.ts

@@ -703,6 +703,72 @@ describe("MessageStore", () => {
     });
   });
 
+  describe("getAllAgentToAgentMessages() / getUnreadAgentToAgentCount()", () => {
+    it("returns newest-first agent-to-agent messages only", () => {
+      store.sendMessage({
+        fromId: "agent-1",
+        fromType: "agent",
+        toId: "agent-2",
+        toType: "agent",
+        content: "first",
+        type: "agent-to-agent",
+      });
+      const second = store.sendMessage({
+        fromId: "agent-2",
+        fromType: "agent",
+        toId: "agent-1",
+        toType: "agent",
+        content: "second",
+        type: "agent-to-agent",
+      });
+      store.sendMessage({
+        fromId: "agent-1",
+        fromType: "agent",
+        toId: "dashboard",
+        toType: "user",
+        content: "not included",
+        type: "agent-to-user",
+      });
+
+      const messages = store.getAllAgentToAgentMessages();
+      expect(messages).toHaveLength(2);
+      expect(messages[0].id).toBe(second.id);
+      expect(messages.every((message) => message.type === "agent-to-agent")).toBe(true);
+    });
+
+    it("counts unread agent-to-agent messages only", () => {
+      const unread = store.sendMessage({
+        fromId: "agent-1",
+        fromType: "agent",
+        toId: "agent-2",
+        toType: "agent",
+        content: "unread",
+        type: "agent-to-agent",
+      });
+      const read = store.sendMessage({
+        fromId: "agent-2",
+        fromType: "agent",
+        toId: "agent-1",
+        toType: "agent",
+        content: "read",
+        type: "agent-to-agent",
+      });
+      store.sendMessage({
+        fromId: "agent-1",
+        fromType: "agent",
+        toId: "dashboard",
+        toType: "user",
+        content: "non-agent",
+        type: "agent-to-user",
+      });
+
+      store.markAsRead(read.id);
+
+      expect(store.getUnreadAgentToAgentCount()).toBe(1);
+      expect(store.getAllAgentToAgentMessages().map((message) => message.id)).toContain(unread.id);
+    });
+  });
+
   describe("events", () => {
     it("emits message:sent event on send", () => {
       const events: Message[] = [];

packages/core/src/message-store.ts

@@ -417,6 +417,32 @@ export class MessageStore extends EventEmitter<MessageStoreEvents> {
     };
   }
 
+  /**
+   * Get all agent-to-agent messages across all agents.
+   * @returns Array of messages (newest first)
+   */
+  getAllAgentToAgentMessages(): Message[] {
+    const rows = this.db.prepare(`
+      SELECT * FROM messages
+      WHERE type = ?
+      ORDER BY createdAt DESC, rowid DESC
+    `).all("agent-to-agent");
+
+    return (rows as unknown as MessageRow[]).map((row) => this.rowToMessage(row));
+  }
+
+  /**
+   * Get unread count across all agent-to-agent messages.
+   */
+  getUnreadAgentToAgentCount(): number {
+    const row = this.db.prepare(`
+      SELECT COUNT(*) as count FROM messages
+      WHERE type = ? AND read = 0
+    `).get("agent-to-agent") as { count: number } | undefined;
+
+    return row?.count ?? 0;
+  }
+
   /**
    * Set or update the hook used when messages are sent to agents.
    */