feat(voice): add cross-call memory and Brave Search tool

- Query user memories at call start and inject into agent system prompt
- Extract new facts after each exchange using claude-haiku via LiteLLM
- Add Brave Search tool (@function_tool) for current data queries
- Pass memory client and caller_user_id through VoiceSession constructor
- Pre-compute 8 HMAC-ratcheted EC keys for reliable E2EE decryption

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot.py

@@ -463,6 +463,8 @@ class Bot:
                         bot_key=bot_key,
                         publish_key_cb=lambda key, rid=room_id: asyncio.ensure_future(
                             self._publish_encryption_key(rid, key)),
+                        memory=self.memory,
+                        caller_user_id=event.sender,
                     )
 
                     # Check timeline for caller's key