feat(routing): route replies to originating channel

Implement reply routing based on OriginatingChannel/OriginatingTo fields.
This ensures replies go back to the provider where the message originated
instead of using the session's lastChannel.

Changes:
- Add OriginatingChannel/OriginatingTo fields to MsgContext (templating.ts)
- Add originatingChannel/originatingTo fields to FollowupRun (queue.ts)
- Create route-reply.ts with provider-agnostic router
- Update all providers (Telegram, Slack, Discord, Signal, iMessage)
  to pass originating channel info
- Update reply.ts to pass originating channel to followupRun
- Update followup-runner.ts to use route-reply for originating channels

This addresses the issue where messages from one provider (e.g., Slack)
would receive replies on a different provider (e.g., Telegram) because
the queue used the last active dispatcher instead of the originating one.

src/telegram/bot.ts

@@ -414,6 +414,7 @@ export function createTelegramBot(opts: TelegramBotOptions) {
       SenderId: senderId || undefined,
       SenderUsername: senderUsername || undefined,
       Provider: "telegram",
+      Surface: "telegram",
       MessageSid: String(msg.message_id),
       ReplyToId: replyTarget?.id,
       ReplyToBody: replyTarget?.body,
@@ -433,6 +434,9 @@ export function createTelegramBot(opts: TelegramBotOptions) {
       CommandAuthorized: commandAuthorized,
       MessageThreadId: messageThreadId,
       IsForum: isForum,
+      // Originating channel for reply routing.
+      OriginatingChannel: "telegram" as const,
+      OriginatingTo: `telegram:${chatId}`,
     };
 
     if (replyTarget && shouldLogVerbose()) {