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/sh/sh.c

@@ -105,7 +105,7 @@ static void builtin_help(void) {
 }
 
 /** Execute an external command via fork+exec. */
-static void run_command(const char *cmd) {
+static void run_command(const char *cmd, const char *arg) {
     int32_t pid = fork();
     if (pid < 0) {
         puts("sh: fork failed\n");
@@ -113,7 +113,14 @@ static void run_command(const char *cmd) {
     }
 
     if (pid == 0) {
-        /* Child: exec the command */
+        /* Child: set ARG1 if there's an argument */
+        if (arg && *arg) {
+            setenv("ARG1", arg);
+        } else {
+            setenv("ARG1", "");
+        }
+
+        /* exec the command */
         int32_t ret = exec(cmd);
         if (ret < 0) {
             puts("sh: ");
@@ -172,7 +179,7 @@ int main(void) {
             builtin_help();
         } else {
             /* External command */
-            run_command(line);
+            run_command(line, arg);
         }
     }