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.

tests/client/group-chat-mention-options.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from 'vitest'
+import { buildMentionOptions } from '@/components/hermes/group-chat/mention-options'
+
+describe('group chat mention options', () => {
+  const agents = [
+    { name: 'Alice', profile: 'alice-profile' },
+    { name: 'Bob', profile: 'bob-profile' },
+    { name: 'all', profile: 'literal-all-agent' },
+  ]
+
+  it('offers @all before agent mentions when the mention query is empty', () => {
+    expect(buildMentionOptions(agents, '').map(option => option.key)).toEqual([
+      'special:all',
+      'agent:Alice',
+      'agent:Bob',
+    ])
+  })
+
+  it('keeps @all reserved when filtering by all and hides a literal all agent', () => {
+    expect(buildMentionOptions(agents, 'all')).toEqual([
+      {
+        key: 'special:all',
+        type: 'all',
+        name: 'all',
+        label: '@all',
+        description: 'All agents',
+      },
+    ])
+  })
+
+  it('filters normal agent mentions without showing @all for unrelated queries', () => {
+    expect(buildMentionOptions(agents, 'bo').map(option => option.key)).toEqual(['agent:Bob'])
+  })
+})