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/reply.ts

@@ -714,6 +714,9 @@ export async function getReplyFromConfig(
     prompt: queuedBody,
     summaryLine: baseBodyTrimmedRaw,
     enqueuedAt: Date.now(),
+    // Originating channel for reply routing.
+    originatingChannel: ctx.OriginatingChannel,
+    originatingTo: ctx.OriginatingTo,
     run: {
       agentId,
       agentDir,