Fix new-window context leak showing previous session ctx%

Stop tmux status from falling back to global status files, seed new
tmux instances at 0%, rank live sessions by opened_at instead of
signals.json keepalive mtime, and bind Grok pids to tmux more reliably.

plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "grok-build-hud",
-  "version": "0.3.12",
+  "version": "0.3.13",
   "description": "第三方 Grok Build 状态条（非官方）：同窗口上下文/配额/token/工具/待办；tmux HUD + 会话斜杠命令",
   "author": {
     "name": "Redredchen01"