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

src/syscall.c

@@ -61,6 +61,11 @@ static int32_t sys_write(registers_t *regs) {
         return (int32_t)len;
     }
 
+    /* VFS file descriptors (fd >= 3) */
+    if (fd >= 3) {
+        return vfs_write(fd, buf, len);
+    }
+
     return -1; /* Invalid fd */
 }
 
@@ -83,6 +88,11 @@ static int32_t sys_read(registers_t *regs) {
         return 0; /* No data available */
     }
 
+    /* VFS file descriptors (fd >= 3) */
+    if (fd >= 3) {
+        return vfs_read(fd, buf, len);
+    }
+
     return -1; /* Invalid fd */
 }
 
@@ -282,6 +292,29 @@ static int32_t sys_setenv(registers_t *regs) {
     return env_set(&cur->env, key, value);
 }
 
+/**
+ * Handle SYS_OPEN: open a file by path.
+ * EBX = path string, ECX = flags.
+ * Returns file descriptor (>= 3) on success, -1 on failure.
+ */
+static int32_t sys_open(registers_t *regs) {
+    const char *path = (const char *)regs->ebx;
+    uint32_t flags = regs->ecx;
+    return (int32_t)vfs_open(path, flags);
+}
+
+/**
+ * Handle SYS_CLOSE: close a file descriptor.
+ * EBX = fd.
+ * Returns 0 on success, -1 on failure.
+ */
+static int32_t sys_close(registers_t *regs) {
+    int fd = (int)regs->ebx;
+    if (fd < 3) return -1;  /* Don't close stdin/stdout/stderr */
+    vfs_close(fd);
+    return 0;
+}
+
 /**
  * Handle SYS_READDIR: read a directory entry.
  * EBX = path, ECX = index, EDX = name buffer (128 bytes min).
@@ -320,6 +353,8 @@ static syscall_fn syscall_table[NUM_SYSCALLS] = {
     [SYS_GETENV]  = sys_getenv,
     [SYS_SETENV]  = sys_setenv,
     [SYS_READDIR] = sys_readdir,
+    [SYS_OPEN]    = sys_open,
+    [SYS_CLOSE]   = sys_close,
 };
 
 void syscall_handler(registers_t *regs) {