Refactor tool-speechtotext: extract sttlib shared library and add tests

Extract duplicated code (Whisper loading, audio recording, transcription,
VAD processing) into reusable sttlib/ package. Rewrite all 3 scripts as
thin wrappers. Add 24 unit tests with mocked hardware. Fix GPU fallback
bug in assistant.py and args.system assignment bug.

python/tool-speechtotext/sttlib/transcription.py

@@ -0,0 +1,19 @@
+HALLUCINATION_PATTERNS = [
+    "thank you", "thanks for watching", "subscribe",
+    "bye", "the end", "thank you for watching",
+    "please subscribe", "like and subscribe",
+]
+
+
+def transcribe(model, audio_float32):
+    """Transcribe audio using Whisper. Returns stripped text."""
+    segments, _ = model.transcribe(audio_float32, beam_size=5)
+    return "".join(segment.text for segment in segments).strip()
+
+
+def is_hallucination(text):
+    """Return True if text looks like a Whisper hallucination."""
+    lowered = text.lower().strip()
+    if len(lowered) < 3:
+        return True
+    return any(p in lowered for p in HALLUCINATION_PATTERNS)