feat: route voice requests through OpenClaw agent session

- Remove local Conversation history (now managed by gateway)
- Use x-openclaw-session-key for persistent agent sessions
- Agent now has full context: SOUL.md, MEMORY.md, tools
- Add VOICE_SESSION_KEY env var (default: agent:main:voice:home)
- Backward compatible: conv parameter kept for compatibility

.env.example

@@ -34,3 +34,5 @@ FOLLOWUP_TIMEOUT=8
 
 # Логирование
 LOG_FILE=errors.log
+
+VOICE_SESSION_KEY=agent:main:voice:home