feat(agent): add crash logs and watchdog startup

- Recover panics to disk for diagnostics
- Add 5-minute watchdog task with startup
- Add -ensure flag to start detached agent
- Add script to pull remote folder via API

lib/crashlog/crashlog.go

@@ -0,0 +1,58 @@
+package crashlog
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime/debug"
+	"sync"
+	"time"
+
+	"tea.chunkbyte.com/kato/go-worm/lib/config"
+)
+
+var mu sync.Mutex
+
+// Recover logs a panic and re-raises after writing the crash file.
+func Recover() {
+	if r := recover(); r != nil {
+		_ = write("panic", fmt.Sprint(r), debug.Stack())
+		panic(r)
+	}
+}
+
+// RecordPanic writes a recovered panic without exiting (e.g. HTTP handlers).
+func RecordPanic(recovered any) {
+	_ = write("panic", fmt.Sprint(recovered), debug.Stack())
+}
+
+// RecordError writes a fatal error before exit.
+func RecordError(err error) {
+	if err == nil {
+		return
+	}
+	_ = write("fatal", err.Error(), debug.Stack())
+}
+
+func write(kind, detail string, stack []byte) error {
+	dir, err := config.LogsDir()
+	if err != nil {
+		return err
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return err
+	}
+	name := fmt.Sprintf("crash-%s.log", time.Now().Format("2006-01-02-150405"))
+	path := filepath.Join(dir, name)
+	body := fmt.Sprintf(
+		"time: %s\nversion: %s\nkind: %s\ndetail: %s\n\nstack:\n%s",
+		time.Now().Format(time.RFC3339Nano),
+		config.Version,
+		kind,
+		detail,
+		stack,
+	)
+	return os.WriteFile(path, []byte(body), 0o600)
+}