feat: make DEFAULT_SYSTEM_PROMPT configurable via environment variable

[Copilot]

System prompt was hardcoded in src/rag.py line 27. While tenant-level override worked, the global default was not configurable.

Changes

• src/config.py: Add default_system_prompt field to LLMConfig (env: LLM_DEFAULT_SYSTEM_PROMPT)
• src/rag.py: Read from config.llm.default_system_prompt instead of hardcoded string
• src/multi_tenant.py: Use config value as fallback for constructor parameter
• env.example: Document LLM_DEFAULT_SYSTEM_PROMPT option

Usage

# Override default system prompt (optional)
LLM_DEFAULT_SYSTEM_PROMPT="Your custom system prompt here"

Default value unchanged: "You are a helpful assistant. Provide direct answers without showing your reasoning process."

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Default system prompt hardcoded</issue_title>
<issue_description>## 问题描述
系统提示词硬编码。

受影响的文件

• src/rag.py 行 27

问题

虽然可通过租户配置覆盖，但全局默认值硬编码。

解决方案

应该配置：DEFAULT_SYSTEM_PROMPT 环境变量</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Default system prompt hardcoded #23

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