docs: rewrite governance templates into human-readable prose

[Pigbibi]

Summary

• README.md / README.zh-CN.md: moved the templated "QSL architecture role" bullet list out of the opening lines and replaced it with a 2-4 sentence natural-language introduction (same facts, no new claims); the list now lives as its own mid-document section. The Chinese-README link and the investing-risk disclaimer are preserved.
• CONTRIBUTING.md / SECURITY.md: removed the duplicated "## 中文摘要" meta-description block (identical boilerplate across all QSL sibling repos except the repo name). The one risk note worth keeping ("live trading/credential/Cloud Run/broker changes need test or dry-run verification first") was folded into CONTRIBUTING.md's Ground Rules as a normal bullet.
• SECURITY.md's actual security content — vulnerability reporting, secret/credential exposure steps, scope notes — is untouched, word for word.

No code changes.

Additional finding (not fixed, out of the requested scope)

CODE_OF_CONDUCT.md has the same "## 中文摘要" meta-block pattern as CONTRIBUTING.md/SECURITY.md did. Left as-is since it wasn't in the requested file list; flagging in case a follow-up PR should cover it too.

Test plan

• Diff-reviewed each file to confirm no SECURITY.md procedural content was dropped
• Confirmed README facts (layer/responsibility/owns/consumes/must-not) are unchanged, just relocated
• N/A — documentation only, no code/tests affected

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