feat: same-phone mode with echo detection and configurable marker

Adds full support for self-messaging setups where you chat with yourself
and an AI assistant replies in the same WhatsApp bubble.

Changes:
- Same-phone mode (from === to) always allowed, bypasses allowFrom check
- Echo detection via bounded Set (max 100) prevents infinite loops
- Configurable samePhoneMarker in config (default: "[same-phone]")
- Messages prefixed with marker so assistants know the context
- fromMe filter removed from inbound.ts (echo detection in auto-reply)
- Verbose logging for same-phone detection and echo skips

Tests:
- Same-phone allowed without/despite allowFrom configuration
- Body prefixed only when from === to
- Non-same-phone rejected when not in allowFrom

src/web/inbound.ts

@@ -70,7 +70,7 @@ export async function monitorWebInbox(options: {
       // De-dupe on message id; Baileys can emit retries.
       if (id && seen.has(id)) continue;
       if (id) seen.add(id);
-      if (msg.key?.fromMe) continue;
+      // Note: not filtering fromMe here - echo detection happens in auto-reply layer
       const remoteJid = msg.key?.remoteJid;
       if (!remoteJid) continue;
       // Ignore status/broadcast traffic; we only care about direct chats.