Add sysfs VFS driver, SYS_OPEN/CLOSE syscalls, cat app

Sysfs:
- New VFS driver mounted at /sys that lets kernel drivers expose
  virtual text files via namespace registration
- Drivers call sysfs_register(name, ops, ctx) with list/read/write
  callbacks for their namespace
- IDE driver registers 'ide' namespace exposing per-device attributes:
  model, type, channel, drive, sectors, sector_size
- Tested: ls /sys -> ide, ls /sys/ide -> hdd1 cd1,
  cat /sys/ide/hdd1/model -> QEMU HARDDISK

Syscalls:
- Added SYS_OPEN (11) and SYS_CLOSE (12) for file I/O from userspace
- Extended SYS_READ/SYS_WRITE to handle VFS file descriptors (fd >= 3)
- Updated userspace syscalls.h with open()/close() wrappers

Apps:
- New 'cat' app: reads and displays file contents via open/read/close
- Updated 'ls' to accept path argument via ARG1 env var
- Updated shell to pass ARG1 env var to external commands

apps/ls/ls.c

@@ -10,12 +10,14 @@
 #include "syscalls.h"
 
 int main(void) {
-    /* Get the current working directory */
+    /* Check for an explicit path argument first */
     char cwd[128];
-    if (getenv("CWD", cwd, sizeof(cwd)) < 0) {
-        /* Default to root if CWD not set */
-        cwd[0] = '/';
-        cwd[1] = '\0';
+    if (getenv("ARG1", cwd, sizeof(cwd)) < 0 || cwd[0] == '\0') {
+        /* No argument; use the current working directory */
+        if (getenv("CWD", cwd, sizeof(cwd)) < 0) {
+            cwd[0] = '/';
+            cwd[1] = '\0';
+        }
     }
 
     char name[128];