fix: harden pi maintenance lifecycle

tools/thothctl/internal/pi/process_unix.go

@@ -0,0 +1,16 @@
+//go:build !windows
+
+package pi
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func processAlive(pid int) bool {
+	process, err := os.FindProcess(pid)
+	if err != nil { return false }
+	err = process.Signal(syscall.Signal(0))
+	return err == nil || errors.Is(err, syscall.EPERM)
+}