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.

README.md

@@ -46,6 +46,7 @@ nix run git+https://code.m3ta.dev/m3tam3re/nixpkgs#zellij-ps
 | `mem0`             | AI memory assistant with vector storage |
 | `msty-studio`      | Msty Studio application               |
 | `pomodoro-timer`   | Pomodoro timer utility                |
+| `stt-ptt`          | Push to Talk Speech to Text           |
 | `tuxedo-backlight` | Backlight control for Tuxedo laptops  |
 | `zellij-ps`        | Project switcher for Zellij           |