feat(guild): translate <@user.name:NAME> -> <@userId>

Before persist/parse, resolve <@user.name:NAME> (outside backticks) via
Center and rewrite to <@userId>; unresolved tokens left as-is. Translated
ids then flow into the existing mention/wakeup/sub-frame logic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/common/center-auth.ts

@@ -25,3 +25,25 @@ export async function introspectGuildToken(token: string): Promise<{ active: boo
     user: data.user,
   };
 }
+
+// Resolve <@user.name:NAME> names to userIds within this guild node via
+// Center. Unresolved names are simply absent from the returned map.
+export async function resolveUserNames(names: string[]): Promise<Record<string, string>> {
+  const centerBaseUrl = process.env.FABRIC_BACKEND_GUILD_CENTER_BASE_URL;
+  const guildNodeId = process.env.FABRIC_BACKEND_GUILD_NODE_ID;
+  const centerApiKey = process.env.FABRIC_BACKEND_GUILD_CENTER_API_KEY;
+  if (!centerBaseUrl || !guildNodeId || !centerApiKey || !names.length) return {};
+
+  try {
+    const res = await fetch(`${centerBaseUrl}/api/auth/resolve-names`, {
+      method: 'POST',
+      headers: { 'content-type': 'application/json', 'x-api-key': centerApiKey },
+      body: JSON.stringify({ guildNodeId, names }),
+    });
+    if (!res.ok) return {};
+    const data = (await res.json()) as { resolved?: Record<string, string> };
+    return data.resolved ?? {};
+  } catch {
+    return {};
+  }
+}