feat: add Fn hotkey monitor and probe CLI

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Sources/SstProbe/main.swift

@@ -0,0 +1,9 @@
+import Foundation
+
+let command = CommandLine.arguments.dropFirst().first ?? "help"
+switch command {
+case "hotkey":
+    runHotkeyProbe()
+default:
+    print("usage: sst-probe hotkey")
+}