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.

apps/libc/crt0.S

@@ -0,0 +1,21 @@
+/**
+ * @file crt0.S
+ * @brief C runtime startup for user-mode applications.
+ *
+ * Calls the C main() function and then exits with the return value.
+ */
+.section .text
+.global _start
+.extern main
+
+_start:
+    /* Call main() */
+    call main
+
+    /* Exit with main's return value (in EAX) */
+    movl %eax, %ebx     /* exit code = return value */
+    movl $0, %eax        /* SYS_EXIT = 0 */
+    int $0x80
+
+    /* Should never reach here */
+1:  jmp 1b