implement shell command interface and add basic commands (echo, ls, pwd, cd, touch, cat, rm) with syscall integration

src/shell/commands/cat.ts

@@ -0,0 +1,15 @@
+import { IShellCommand } from "./IShellCommand";
+import { ShellSyscall } from "../ShellSyscall";
+
+export class cat implements IShellCommand {
+    constructor(private syscall: ShellSyscall) {}
+
+    getName(): string {
+        return "cat";
+    }
+
+    execute(args: string[]): string {
+        if (args.length === 0) return "cat: missing file operand";
+        return this.syscall.readFile(args[0]);
+    }
+}