feat: shell (sh) with keyboard driver, SYS_READ, SYS_EXEC

- PS/2 keyboard driver: IRQ1 handler, scancode set 1 translation,
  ring buffer, shift key support
- SYS_READ (fd=0): non-blocking keyboard read for stdin
- SYS_YIELD: calls schedule_tick directly (no nested INT 0x80)
- SYS_EXEC: loads binary from CPIO initrd, replaces process image
- User-space libc: crt0.S (C runtime startup), syscalls.h (inline
  syscall wrappers, basic string functions)
- Shell app (sh): readline with echo/backspace, builtins (cd, env,
  help, exit), fork+exec for external commands
- Updated build_apps.sh: C app support with crt0 linking, libc
  include path, .bss section in objcopy

Tested: shell boots, keyboard input works, hello-world runs via
fork+exec, env shows CWD, exit cleanly terminates.

src/CMakeLists.txt

@@ -20,6 +20,7 @@ add_executable(kernel
     vfs.c
     initrd_fs.c
     env.c
+    keyboard.c
     interrupts.S
     kernel.c
 )