Phase 1-2: Migrate to Deno with npm: and node: specifiers

- Created deno.json configuration
- Updated all imports to use npm:net-snmp@3.20.0
- Changed all Node.js built-in imports to node: specifiers
- Updated all .js extensions to .ts in imports
- Created mod.ts as Deno entry point
- Ready for Phase 3: CLI simplification

mod.ts

@@ -0,0 +1,44 @@
+#!/usr/bin/env -S deno run --allow-all
+
+/**
+ * NUPST - UPS Shutdown Tool
+ *
+ * A command-line tool for monitoring SNMP-enabled UPS devices and
+ * initiating system shutdown when power conditions are critical.
+ *
+ * Required Permissions:
+ * - --allow-net: SNMP communication with UPS devices
+ * - --allow-read: Read configuration files (/etc/nupst/config.json)
+ * - --allow-write: Write configuration files
+ * - --allow-run: Execute system commands (systemctl, shutdown, git, bash)
+ * - --allow-sys: Access system information (hostname, OS details)
+ * - --allow-env: Read environment variables
+ *
+ * @module
+ */
+
+import { NupstCli } from './ts/cli.ts';
+
+/**
+ * Main entry point for the NUPST application
+ * Parses command-line arguments and executes the requested command
+ */
+async function main(): Promise<void> {
+  const cli = new NupstCli();
+
+  // Deno.args is already 0-indexed (unlike Node's process.argv which starts at index 2)
+  // We need to prepend placeholder args to match the existing CLI parser expectations
+  const args = ['deno', 'mod.ts', ...Deno.args];
+
+  await cli.parseAndExecute(args);
+}
+
+// Execute main and handle errors
+if (import.meta.main) {
+  try {
+    await main();
+  } catch (error) {
+    console.error(`Error: ${error instanceof Error ? error.message : String(error)}`);
+    Deno.exit(1);
+  }
+}