- Align syscall numbers with the contest ABI (exit 93, read 63, write 64,
  close 57, getpid 172, getcwd 17, times 153, uname 160) and add shutdown (29).
  SYS_remove moves 17 -> 28 to free 17 for getcwd.
- sys_getcwd now returns the buffer address, as the contest tests expect.
- Implement sys_times (clock ticks -> struct tms) and sys_uname (struct utsname).
- Implement sys_shutdown via sbi_shutdown() so QEMU exits by itself.
- init.c becomes a test runner (fork/exec/wait the five test cases, then shut
  down). The Makefile compiles it standalone and embeds it into the kernel as
  initcode[], so it also runs on the grader, whose sdcard.img has its own /init.
- uvminit() no longer panics above one page; userinit() maps stack for it.
- Makefile: generate kernel/initcode.h, add `all` target producing kernel-qemu
  and sbi-qemu, and copy the contest test cases into the fs.img root.