docs: rewrite governance templates into human-readable prose

[Pigbibi]

Summary

• README.md / README.zh-CN.md: replaced the bare "## QSL architecture role" field list at the top with a 2-4 sentence plain-language introduction to what this repo does; the original field list is preserved verbatim, just moved to a mid-document section.
• CONTRIBUTING.md, SECURITY.md, CODE_OF_CONDUCT.md: removed the templated "## 中文摘要" metadata block (identical across all QSL repos, only the repo name swapped).
• CONTRIBUTING.md: the one non-redundant point from its summary block (verify live-trading/credential/permission/Cloud Run/exchange-API changes in a test environment or dry run first) was folded into "Ground Rules" as a normal bullet instead of being dropped.
• SECURITY.md and CODE_OF_CONDUCT.md: the summary's risk-reminder content was already fully covered by the existing body sections (Secret and Credential Exposure; Our Standards / Project Scope), so nothing else needed to move.

Test plan

• Doc-only change; no code touched.
• Reviewed resulting Markdown renders correctly (headings, links, code fences intact).
• Confirmed no other file in the repo still contains the "中文摘要" block or the raw architecture-role list.

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