Add CPIO initial ramdisk with build infrastructure and parser (AI)

Build system changes:
- scripts/gen_initrd.sh packs all files from apps/ into a newc-format
  CPIO archive at build/isodir/boot/initrd.cpio.
- CMakeLists.txt adds 'initrd' target as ISO dependency. GRUB loads the
  archive as a Multiboot2 module via 'module2 /boot/initrd.cpio'.
- apps/README added as placeholder file for initial ramdisk content.

Kernel changes:
- kernel.c scans Multiboot2 tags for MULTIBOOT_TAG_TYPE_MODULE to find
  the initrd's physical address range, then passes it to cpio_init().
- cpio.c/h implements a parser for the SVR4/newc CPIO format:
  - cpio_init(): lists archive contents on startup
  - cpio_find(): look up a file by name (handles ./ prefix)
  - cpio_next(): iterate through all entries
  - cpio_count(): count files in archive
- The initrd lives in identity-mapped physical memory, so no additional
  mapping is needed to access it.

Verified in QEMU: GRUB loads the module at 0x0014A000, CPIO parser
finds the README file (38 bytes). All existing functionality (Ring 3
processes, syscalls) continues to work.

CMakeLists.txt

@@ -19,15 +19,26 @@ add_subdirectory(src)
 file(MAKE_DIRECTORY ${CMAKE_SOURCE_DIR}/release)
 file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/isodir/boot/grub)
 
-# Create grub.cfg for ISO
-file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" { multiboot2 /boot/kernel.bin }")
+# Generate CPIO initial ramdisk from apps directory.
+# All files in apps/ are packed into a newc-format CPIO archive.
+set(INITRD_FILE ${CMAKE_BINARY_DIR}/isodir/boot/initrd.cpio)
+add_custom_command(
+    OUTPUT ${INITRD_FILE}
+    COMMAND ${CMAKE_SOURCE_DIR}/scripts/gen_initrd.sh ${CMAKE_SOURCE_DIR}/apps ${INITRD_FILE}
+    DEPENDS ${CMAKE_SOURCE_DIR}/apps
+    COMMENT "Generating CPIO initial ramdisk"
+)
+add_custom_target(initrd DEPENDS ${INITRD_FILE})
+
+# Create grub.cfg for ISO - includes module2 for the initrd
+file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" { multiboot2 /boot/kernel.bin\n  module2 /boot/initrd.cpio }")
 
 
 # ISO Generation
 add_custom_target(iso ALL
     COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/kernel ${CMAKE_BINARY_DIR}/isodir/boot/kernel.bin
     COMMAND grub-mkrescue -o ${CMAKE_SOURCE_DIR}/release/claude-os.iso ${CMAKE_BINARY_DIR}/isodir
-    DEPENDS kernel
+    DEPENDS kernel initrd
     COMMENT "Generating bootable ISO image"
 )