docs: Contributor Workflow 补充认领独占与超时释放规则

[modusensus]

贡献流程此前只写了「Claim before you start」，缺两处明确规则：

• 认领独占：认领评论即锁定，同 issue 的第二个 PR 标记 duplicate 关闭（防双人撞车重复 PR，也挡掉部分看 label 乱提的流量）；
• 认领超时释放：2 周无 PR 即释放，可重新认领（防占坑不干活）。

纯文档改动，无代码影响。

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