refactor: remove all #if x86 guards from shell.c, migrate to kprintf + HAL

refactor: remove all #if x86 guards from shell.c, migrate to kprintf + HAL

shell.c is generic kernel code and should not contain any
architecture-specific guards or inline assembly.

Changes:
- Replace all uart_print() calls with kprintf() so output goes
  through the console subsystem and into the kernel log buffer
- Replace x86 'cli; ud2' panic asm with hal_cpu_disable_interrupts()
  + hal_cpu_idle() loop
- Replace #if x86 ring3 guard with arch_platform_usermode_test_start()
  which is already a generic wrapper with per-arch implementations
- Use console_write() for echo/backspace (no log buffer pollution)
- Remove unnecessary uart_console.h and vga_console.h includes

Passes: make, cppcheck, QEMU smoke test.