docs(security): 消费共享边界模型并薄化本地文档

[xiaoland]

结果

完成独立 shared-security-model-ownership correction 的 Spoke phase：core-py 消费已合并的 Hub security boundary model，并只保留 runtime-specific realization。

Hub owner：InKCre/docs #19

提交边界

1. docs(shared): consume project security model
   • 只移动 docs/_shared 到 Hub merge commit ecfe286
2. docs(security): keep core runtime boundaries local
   • 将原 216 行混合模型薄化为 core runtime realization note
   • 更新 README、SECURITY、AGENTS 与本地 docs navigation

Ownership 结果

• Hub：actors、assets、trust boundaries、classification、proportionality method
• core-py：FastAPI/PostgREST、database Peer admission、in-process Extension、runtime/provider/delivery realization
• concrete Extension credential：对应 Extension Unit 文档
• vulnerability reporting：根 SECURITY.md

明确不做

• 不改变 runtime/auth/database behavior
• 不增加安全 control、audit、test 或 verifier
• 不复制 Hub 正文到本地

验证

• canonical shared-doc pre-commit check
• git diff --check
• pdm run check：7 passed，40 skipped；Ruff/Pyrefly 通过
• stale ownership wording scan