Add samePhoneResponsePrefix config option

Automatically prefixes responses with a configurable string when in
same-phone mode. This helps distinguish bot replies from user messages
in the same chat bubble.

Example config:
  "samePhoneResponsePrefix": "🦞"

Will prefix all same-phone replies with the lobster emoji.

src/config/config.ts

@@ -47,6 +47,7 @@ export type WarelayConfig = {
   inbound?: {
     allowFrom?: string[]; // E.164 numbers allowed to trigger auto-reply (without whatsapp:)
     samePhoneMarker?: string; // Prefix for same-phone mode messages (default: "[same-phone]")
+    samePhoneResponsePrefix?: string; // Prefix auto-added to replies in same-phone mode (e.g., "🦞")
     transcribeAudio?: {
       // Optional CLI to turn inbound audio into text; templated args, must output transcript to stdout.
       command: string[];
@@ -141,6 +142,7 @@ const WarelaySchema = z.object({
     .object({
       allowFrom: z.array(z.string()).optional(),
       samePhoneMarker: z.string().optional(),
+      samePhoneResponsePrefix: z.string().optional(),
       transcribeAudio: z
         .object({
           command: z.array(z.string()),