feat: add stt-ptt package for voice-to-text and text-to-speech

Implements speech-to-text (via whisper-cpp) and text-to-speech (via espeak)
functionality with key bindings. Replaces coreutils with busybox for lighter
dependencies and removes explicit buildInputs since all paths are hardcoded.

pkgs/default.nix

@@ -7,6 +7,7 @@
   mem0 = pkgs.callPackage ./mem0 {};
   msty-studio = pkgs.callPackage ./msty-studio {};
   pomodoro-timer = pkgs.callPackage ./pomodoro-timer {};
+  stt-ptt = pkgs.callPackage ./stt-ptt {};
   tuxedo-backlight = pkgs.callPackage ./tuxedo-backlight {};
   zellij-ps = pkgs.callPackage ./zellij-ps {};
 }