docs: rewrite governance templates into human-readable prose

[Pigbibi]

Summary

• README.md / README.zh-CN.md: replaced the opening raw "QSL architecture role" / "QSL 架构角色" field list with a short natural-language introduction (role, what it does, who depends on it, and its boundary); moved the original field list unchanged to a mid-document section further down.
• CONTRIBUTING.md, SECURITY.md, CODE_OF_CONDUCT.md: removed the templated "## 中文摘要" metadata block (identical boilerplate repeated across the QuantStrategyLab repos, only the repo name differed).
• CONTRIBUTING.md: folded the one substantive risk note from the removed summary ("verify live-trading/secret/permission/Cloud Run/exchange-broker-API changes in a test environment or dry run before touching production") into the existing Ground Rules list so it isn't lost.
• SECURITY.md and CODE_OF_CONDUCT.md: the removed blocks were pure restatements of content already covered in the body, so nothing else was added back.

No code, workflow, or configuration changes; documentation-only.

Test plan

• Manual read-through of all five changed files for structure and factual accuracy (no facts changed, only reorganized/removed boilerplate).
• Confirmed no other file in the repo still contains a "中文摘要" block (grep -rl "中文摘要" . --include="*.md" returns nothing).
• N/A — no code changes, so no test suite run.

🤖 Generated with Claude Code