docs(phase-07): add/update security threat verification

.planning/phases/07-phase-06-mention-upload/07-SECURITY.md

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+---
+phase: 07
+slug: phase-06-mention-upload
+status: verified
+threats_open: 0
+asvs_level: 1
+created: 2026-04-17
+---
+
+# Phase 07 — Security
+
+> Per-phase security contract: threat register, accepted risks, and audit trail.
+
+---
+
+## Trust Boundaries
+
+| Boundary | Description | Data Crossing |
+|----------|-------------|---------------|
+| 输入框展示态 -> 提交态 payload | 同一条用户消息在展示与提交存在双态，需防止内部提示文案泄露到用户可见区 | 用户原文、拼接提示文本、附件/Skill 标识 |
+| 前端组装器 -> 后端存档消息 | 拼接文案进入提交链路并可能回流，需要保证展示层过滤与提交层分离 | 提交消息正文、`additional_kwargs.files`、历史消息渲染内容 |
+
+---
+
+## Threat Register
+
+| Threat ID | Category | Component | Disposition | Mitigation | Status |
+|-----------|----------|-----------|-------------|------------|--------|
+| T-07-01 | I (Information Disclosure) | `frontend/src/core/threads/hooks.ts` + `frontend/src/components/workspace/messages/message-list-item.tsx` | mitigate | 提交态使用 `submitText`，展示态经 `stripPriorityHintSuffix` 过滤；E2E 验证消息区不回显优先提示 | closed |
+| T-07-02 | T (Tampering / flow bypass) | `frontend/src/components/workspace/input-box.tsx` | mitigate | 发送入口统一经 `requestSubmit -> handleSubmit` 透传 references/skills，避免分支漏传 | closed |
+| T-07-03 | R (Repudiation / traceability) | `frontend/tests/e2e/input-and-compose.spec.ts` | mitigate | 增加请求拦截断言（DF-INPUT-008A），可审计提交内容含 `XClaw优先使用` 且 UI 不显示后缀 | closed |
+
+*Status: open · closed*  
+*Disposition: mitigate (implementation required) · accept (documented risk) · transfer (third-party)*
+
+---
+
+## Accepted Risks Log
+
+No accepted risks.
+
+---
+
+## Security Audit Trail
+
+| Audit Date | Threats Total | Closed | Open | Run By |
+|------------|---------------|--------|------|--------|
+| 2026-04-17 | 3 | 3 | 0 | Codex (`/gsd-secure-phase 7`) |
+
+---
+
+## Sign-Off
+
+- [x] All threats have a disposition (mitigate / accept / transfer)
+- [x] Accepted risks documented in Accepted Risks Log
+- [x] `threats_open: 0` confirmed
+- [x] `status: verified` set in frontmatter
+
+**Approval:** verified 2026-04-17