feat(group-chat): add @all mention routing (#857)

Add modular group-chat mention routing helpers for the reserved @all token, route it to every non-sender agent, and strip routing tokens before model input.

Expose @all in mention autocomplete, highlight it in group messages, reserve literal all agent names, and cover boundary/partial-match regressions with tests.

packages/server/src/routes/hermes/group-chat.ts

@@ -1,5 +1,6 @@
 import Router from '@koa/router'
 import type { GroupChatServer } from '../../services/hermes/group-chat'
+import { isReservedMentionName } from '../../services/hermes/group-chat/mention-routing'
 
 export const groupChatRoutes = new Router()
 
@@ -45,6 +46,12 @@ groupChatRoutes.post('/api/hermes/group-chat/rooms', async (ctx) => {
         ctx.body = { error: 'name and inviteCode are required' }
         return
     }
+    const reservedAgent = (agents || []).find(a => isReservedMentionName(a.name || a.profile))
+    if (reservedAgent) {
+        ctx.status = 400
+        ctx.body = { error: '`all` is reserved for @all mentions' }
+        return
+    }
 
     const roomId = generateId()
     const storage = chatServer.getStorage()
@@ -213,6 +220,11 @@ groupChatRoutes.post('/api/hermes/group-chat/rooms/:roomId/agents', async (ctx)
         ctx.body = { error: 'profile is required' }
         return
     }
+    if (isReservedMentionName(name || profile)) {
+        ctx.status = 400
+        ctx.body = { error: '`all` is reserved for @all mentions' }
+        return
+    }
 
     // Prevent duplicate agent in same room
     const existing = chatServer.getStorage().getRoomAgents(ctx.params.roomId)