feat(max-bot): implement mock-based message flow with WIP file queue

- MAX API integration (long polling)
- MockPlatformClient for agent simulation
- File download & workspace storage
- Basic commands: /help, /start
- Attachment queue: add works, list/remove need testing

[WIP: attachment queue commands]
[MOCK-ONLY: requires real agent for production]

.env.example

@@ -34,4 +34,5 @@ SURFACES_BOT_STATE_VOLUME=surfaces-bot-state
 # MAX Surface
 MAX_BOT_TOKEN=real_max_token
 MAX_API_URL=https://platform-api.max.ru
-MAX_AGENT_REGISTRY_PATH=/app/config/max-agents.yaml
+MAX_AGENT_REGISTRY_PATH=/app/config/max-agents.yaml
+MAX_PLATFORM_BACKEND=mock  # ← ДОБАВИТЬ: "mock" для локалки, "real" для прода