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.

package.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 xAI 官方）：同窗口上下文/配额/token/工具；plugin + CLI + tmux HUD",
   "type": "module",
   "main": "dist/src/index.js",