docs: add contributing, security, and code of conduct guides

[kittimzhe]

Adds bilingual (EN/中文) community governance documents:

• CONTRIBUTING.md — issue reporting flow (versions + repro required), PR norms (one concern per PR, conventional commits, tests for behavior changes, EN/ZH README sync rule), dev setup commands.
• SECURITY.md — private vulnerability disclosure via GitHub Security Advisories (no public issues), 72h acknowledgement, supported-versions policy.
• CODE_OF_CONDUCT.md — Contributor Covenant v2.1 adapted, maintainer enforcement contact.

README.md / README.zh.md gain a Community / 社区 section linking all three (near License).

No code changes.