Replace the fixed 10s post-wake-word listen window with a VAD-driven
loop. Once at least 500ms of speech have been seen, 1.2s of silence
closes the window. Hard cap at 15s as a safety net.
Tunable in config.py via VAD_AGGRESSIVENESS, COMMAND_END_SILENCE_MS,
COMMAND_MIN_SPEECH_MS, COMMAND_MIN_LISTEN_MS, COMMAND_MAX_LISTEN_MS.
uses a second KaldiRecognizer with a JSON grammar of WAKE_WORDS + [unk] for
the wake phase, then hands off to the full-vocab recognizer for the 10s
command window. drops the pvporcupine dependency entirely.
