docs: add CONTRIBUTING.md, SECURITY.md, and CODE_OF_CONDUCT.md

Add three community health files with bilingual (EN/ZH) support:
- CODE_OF_CONDUCT.md: Contributor Covenant v2.1 with official Chinese translation
- SECURITY.md: security policy pointing to GitHub Security Advisories
- CONTRIBUTING.md: contribution guide with dev setup, code style, PR guidelines,
  i18n rules, and AI-assisted contribution policy

SECURITY.md

@@ -0,0 +1,58 @@
+# Security Policy / 安全策略
+
+## Supported Versions / 支持的版本
+
+Only the latest release of CC Switch receives security updates.
+
+仅最新版本的 CC Switch 会收到安全更新。
+
+| Version / 版本 | Supported / 是否支持 |
+|----------------|---------------------|
+| Latest 3.x     | ✅ Yes / 是          |
+| < 3.0          | ❌ No / 否           |
+
+## Reporting a Vulnerability / 报告漏洞
+
+**Please do NOT report security vulnerabilities through public GitHub issues.**
+
+**请不要通过公开的 GitHub Issue 报告安全漏洞。**
+
+Instead, please report them through [GitHub Security Advisories](https://github.com/farion1231/cc-switch/security/advisories/new).
+
+请通过 [GitHub 安全公告](https://github.com/farion1231/cc-switch/security/advisories/new) 进行报告。
+
+When reporting, please include:
+
+报告时请包含以下信息：
+
+- A description of the vulnerability / 漏洞描述
+- Steps to reproduce / 复现步骤
+- Potential impact / 潜在影响
+- Affected versions / 受影响版本
+
+## Response Timeline / 响应时间
+
+- **Acknowledgment / 确认**: within 48 hours / 48 小时内
+- **Initial assessment / 初步评估**: within 7 days / 7 天内
+- **Fix for critical issues / 关键问题修复**: within 14 days / 14 天内
+
+## Disclosure Policy / 披露政策
+
+We follow a coordinated disclosure process:
+
+我们遵循协调披露流程：
+
+1. The reporter submits the vulnerability privately. / 报告者私下提交漏洞。
+2. We confirm and work on a fix. / 我们确认并修复漏洞。
+3. A patch release is published. / 发布修复版本。
+4. The vulnerability is publicly disclosed. / 公开披露漏洞详情。
+
+Reporters will be credited in the release notes unless they prefer to remain anonymous.
+
+除非报告者希望匿名，否则将在发布说明中致谢。
+
+## Security Updates / 安全更新
+
+Security fixes are released as patch versions and announced via [GitHub Releases](https://github.com/farion1231/cc-switch/releases). We recommend always updating to the latest version.
+
+安全修复通过补丁版本发布，并通过 [GitHub Releases](https://github.com/farion1231/cc-switch/releases) 通知。建议始终更新到最新版本。