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/auto-reply/templating.ts

@@ -1,3 +1,13 @@
+/** Valid provider channels for message routing. */
+export type OriginatingChannelType =
+  | "telegram"
+  | "slack"
+  | "discord"
+  | "signal"
+  | "imessage"
+  | "whatsapp"
+  | "webchat";
+
 export type MsgContext = {
   Body?: string;
   From?: string;
@@ -38,6 +48,17 @@ export type MsgContext = {
   MessageThreadId?: number;
   /** Telegram forum supergroup marker. */
   IsForum?: boolean;
+  /**
+   * Originating channel for reply routing.
+   * When set, replies should be routed back to this provider
+   * instead of using lastChannel from the session.
+   */
+  OriginatingChannel?: OriginatingChannelType;
+  /**
+   * Originating destination for reply routing.
+   * The chat/channel/user ID where the reply should be sent.
+   */
+  OriginatingTo?: string;
 };
 
 export type TemplateContext = MsgContext & {