fix(discord): use cached author.id when DMChannel.recipientId is null

DMChannel.recipientId can be null when client.channels.fetch() returns
a DM channel with a cold cache. The inbound gate correctly uses
msg.author.id, but fetchAllowedChannel relied on recipientId, so
replies to allowlisted DMs intermittently failed with "channel not
allowlisted" after session restart.

Maintain a channelId→userId map populated during inbound handling and
fall back to it when recipientId is null.

Fixes anthropics/claude-code#40576
Fixes anthropics/claude-code#41647

🏠 Remote-Dev: homespace

external_plugins/discord/server.ts

@@ -222,6 +222,8 @@ type GateResult =
 const recentSentIds = new Set<string>()
 const RECENT_SENT_CAP = 200
 
+const dmChannelUsers = new Map<string, string>()
+
 function noteSent(id: string): void {
   recentSentIds.add(id)
   if (recentSentIds.size > RECENT_SENT_CAP) {
@@ -404,7 +406,8 @@ async function fetchAllowedChannel(id: string) {
   const ch = await fetchTextChannel(id)
   const access = loadAccess()
   if (ch.type === ChannelType.DM) {
-    if (access.allowFrom.includes(ch.recipientId)) return ch
+    const userId = ch.recipientId ?? dmChannelUsers.get(id)
+    if (userId && access.allowFrom.includes(userId)) return ch
   } else {
     const key = ch.isThread() ? ch.parentId ?? ch.id : ch.id
     if (key in access.groups) return ch
@@ -823,6 +826,10 @@ async function handleInbound(msg: Message): Promise<void> {
 
   const chat_id = msg.channelId
 
+  if (msg.channel.type === ChannelType.DM) {
+    dmChannelUsers.set(chat_id, msg.author.id)
+  }
+
   // Permission-reply intercept: if this looks like "yes xxxxx" for a
   // pending permission request, emit the structured event instead of
   // relaying as chat. The sender is already gate()-approved at this point