feat: add Anthropic API provider support and missing env vars

- Add LLM_PROVIDER env var (openai/anthropic) to switch cloud backend
- Use ChatAnthropic for anthropic provider with custom base_url
- Add CONTEXT_MAX_TOKENS, CONTEXT_KEEP_RECENT, SESSIONS_DIR,
  HISTORY_MAX_SNAPSHOTS to .env and .env.example
- Add langchain-anthropic dependency to requirements.txt

Note: E2E testing blocked — the configured MiniMax API key
(sk-cp-...) returns 401 across all endpoints (Anthropic and OpenAI).
The API key may be expired or lack text-generation model access.

.env.example

@@ -1,6 +1,9 @@
 # 大语言模型后端：cloud 或 local
 LLM_BACKEND=cloud
 
+# 云端提供商：openai 或 anthropic
+LLM_PROVIDER=openai
+
 # 云端配置（OpenAI 兼容）
 OPENAI_API_KEY=sk-xxxx
 OPENAI_BASE_URL=https://api.openai.com/v1