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/imessage/monitor.ts

@@ -351,10 +351,11 @@ export async function monitorIMessageProvider(
           : normalizeIMessageHandle(sender),
       },
     });
+    const imessageTo = chatTarget || `imessage:${sender}`;
     const ctxPayload = {
       Body: body,
       From: isGroup ? `group:${chatId}` : `imessage:${sender}`,
-      To: chatTarget || `imessage:${sender}`,
+      To: imessageTo,
       SessionKey: route.sessionKey,
       AccountId: route.accountId,
       ChatType: isGroup ? "group" : "direct",
@@ -372,6 +373,9 @@ export async function monitorIMessageProvider(
       MediaUrl: mediaPath,
       WasMentioned: mentioned,
       CommandAuthorized: commandAuthorized,
+      // Originating channel for reply routing.
+      OriginatingChannel: "imessage" as const,
+      OriginatingTo: imessageTo,
     };
 
     if (!isGroup) {