feat: hello-world user-mode app loaded from initrd via VFS

- Created apps/hello-world/hello-world.S: AT&T assembly user program that
  calls SYS_WRITE to print 'Hello, World' then SYS_EXIT(0)
- Created apps/user.ld: linker script for user apps at 0x08048000
- Created scripts/build_apps.sh: builds each app dir into flat binary
- Updated CMakeLists.txt: added apps build target in ISO pipeline
- Updated gen_initrd.sh: packs built binaries from build/apps_bin/
- Updated kernel.c: replaced inline machine code with VFS-based loading
  of hello-world from /initrd/hello-world via cpio_find()

Verified: hello-world binary (49 bytes) loads from CPIO initrd,
prints 'Hello, World', and exits with code 0.

apps/user.ld

@@ -0,0 +1,25 @@
+/* Linker script for ClaudeOS user-mode flat binary.
+ * Applications are loaded at 0x08048000 (USER_CODE_START).
+ * This produces a flat binary (no ELF headers). */
+
+ENTRY(_start)
+
+SECTIONS {
+    . = 0x08048000;
+
+    .text : {
+        *(.text)
+    }
+
+    .rodata : {
+        *(.rodata*)
+    }
+
+    .data : {
+        *(.data)
+    }
+
+    .bss : {
+        *(.bss)
+    }
+}