refactor: VAD upgrade, retry, dead code cleanup, AGENT removal

- audio: switch VAD to webrtcvad with RMS gate + fallback to RMS
- audio: honor FOLLOWUP_TIMEOUT — short silence wait after bot response
- llm: retry with exponential backoff on network errors and 5xx
- llm: VOICE_MAX_TOKENS env (default 300) instead of hardcoded 150
- tts: optional VAD-based barge-in (BARGE_IN_ENABLED, off by default)
- tts: remove dead start_barge_in_listener / was_barge_in helpers
- config: drop AGENT/LUSYA_AGENT — routing happens via session_key
- modes: remove unused imports, pass FOLLOWUP_TIMEOUT to follow-up record()
- docs: full rewrite of README and CLAUDE.md to match current architecture

requirements.txt

@@ -6,6 +6,7 @@ numpy<2
 pyaudio
 sounddevice
 scipy<1.15
+webrtcvad-wheels
 
 # STT через облако
 groq