feat: add cwd tracking to kernel, remove shell hack, update system utilities to use getcwd, fix tcc libc stub, fix nonexistent directory issue

programs/include/libc/unistd.h

@@ -13,6 +13,8 @@ int    read(int fd, void *buf, size_t count);
 int    write(int fd, const void *buf, size_t count);
 int    close(int fd);
 long   lseek(int fd, long offset, int whence);
+int    chdir(const char *path);
+char  *getcwd(char *buf, size_t size);
 
 unsigned int sleep(unsigned int seconds);