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/cat/cat.c

@@ -0,0 +1,41 @@
+/**
+ * @file cat.c
+ * @brief Display file contents.
+ *
+ * Reads a file specified as the first argument (from shell)
+ * and prints its contents to stdout. If no argument is given,
+ * prints usage.
+ *
+ * Usage: cat <filepath>
+ */
+
+#include "syscalls.h"
+
+int main(void) {
+    /* Get the file path from the ARGS environment variable.
+     * The shell sets ARGS to the arguments after the command name. */
+    char path[128];
+    if (getenv("ARG1", path, sizeof(path)) < 0) {
+        puts("Usage: cat <file>\n");
+        return 1;
+    }
+
+    /* Open the file */
+    int32_t fd = open(path, 0);
+    if (fd < 0) {
+        puts("cat: ");
+        puts(path);
+        puts(": open failed\n");
+        return 1;
+    }
+
+    /* Read and print in chunks */
+    char buf[256];
+    int32_t n;
+    while ((n = read(fd, buf, sizeof(buf))) > 0) {
+        write(1, buf, (uint32_t)n);
+    }
+
+    close(fd);
+    return 0;
+}