feat(v0.4): add WithLock helper that warns-and-skips on timeout

internal/lockfile/writelock.go

@@ -52,3 +52,20 @@ func Acquire(lockPath string, timeout, staleAfter time.Duration) (func(), error)
 		}
 	}
 }
+
+// WithLock acquires the lock at lockPath, runs fn while holding it, and
+// releases the lock. If the lock cannot be acquired within timeout, fn is
+// NOT called and (skipped=true, err=nil) is returned — this matches the
+// ctask "warn and skip, never hang" contract for metadata writes.
+// Any error returned by fn is surfaced to the caller with skipped=false.
+func WithLock(lockPath string, timeout, staleAfter time.Duration, fn func() error) (skipped bool, err error) {
+	release, err := Acquire(lockPath, timeout, staleAfter)
+	if err != nil {
+		if errors.Is(err, ErrTimeout) {
+			return true, nil
+		}
+		return false, err
+	}
+	defer release()
+	return false, fn()
+}