技术架构讨论：PR #616 local 模式会话认领合并入账号（2352 行核心改动）

## 背景

PR #616 "feat(desktop): local 模式会话在登录后认领并合并入账号" 是一个大型新功能，改动量较大（核心路径 2065 行，总计 2352 行）。

## 触发原因

- core-paths：核心路径改动 2065 行 ≥ 150
- huge-diff：2352 行 ≥ 800

## 需要

技术白名单成员确认：本次架构改动是否可控，设计是否合理。

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*由 review-pr 流程自动创建*
关联 PR: #616

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关联 PR:#616(作者 @zqchris);本 issue 由 review-pr 流程自动创建,用于先讨论该 PR 涉及的技术架构调整,聊清楚后 PR 会恢复审查。

Start by reviewing PR #616 and its 2,352-line diff, focusing on the local-mode session claim and account-merge architecture described here. Done means the technical design is confirmed as controllable and reasonable, or the required architectural concerns are recorded before the PR resumes review.

Written by the indexing model from the issue text.