refactor: route link() through VFS callback — remove last diskfs bypass from syscall.c

refactor: route link() through VFS callback — remove last diskfs bypass from syscall.c

- fs.h: add link callback to fs_node_t (dir, name, target_node)
- fs.c: implement vfs_link() wrapper — resolves old_path to node,
  new_path to parent+basename, calls parent->link()
- diskfs.c: implement diskfs_vfs_link() using parent ino + target ino,
  wire into diskfs_set_dir_ops()
- syscall.c: syscall_link_impl now calls vfs_link() instead of
  extern diskfs_link() with /disk/ prefix stripping

syscall.c no longer references any diskfs symbol.