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/commands/mailbox-status.md

@@ -11,7 +11,7 @@ Print exactly this block, filling in each line:
 Claude-Mailbox status
   binary:       <output of `claude-mailbox --version`, or "not installed">
   daemon:       <output of `claude-mailbox status`>
-  health:       <"ok" if GET http://127.0.0.1:47822/health returns 200, else "unreachable">
+  health:       <"ok" if GET http://127.0.0.1:37849/health returns 200, else "unreachable">
   mailbox name: <value of env.CLAUDE_MAILBOX_NAME in ./.claude/settings.json, or "unset"; also note if ~/.claude/settings.json has a value>
   pending:      <integer count from `claude-mailbox peek --name <resolved-name>` if name is set, else "n/a">
 ```