feat: change default port from 47822 to 37849 (v1.2.0)

47822 collided with ClaudeDo.Worker.exe on at least one user's machine.
37849 is high, registered to nobody, and avoids the prior conflict.
Both the Node port and the .NET port move together (still
wire-compatible). Defaults change only — if a user has a custom port
in mailbox.json, that stays.

plugin/README.md

@@ -14,7 +14,7 @@ The doctor walks the rest:
 
 1. installs the `claude-mailbox` binary via `npm install -g @kuns/claude-mailbox` if missing (asks first)
 2. registers the daemon for autostart and starts it if needed
-3. health-probes `http://127.0.0.1:47822/health`
+3. health-probes `http://127.0.0.1:37849/health`
 4. optionally lets you set a **base prefix** (e.g., `backend`) — without one, mailbox names are anonymous (`claude-XXXXXXXX`)
 5. runs a self → self smoke test