fix(sidechat): stamp the seed with fork markers so the child inherits no pending inbox

sidechat.start passed `seed` without `meta.isSeeded: true` +
`inheritedEventCount`, so dsh-session treated the whole seed as the
child's OWN events and the child's Inbox constructor replayed the
parent's `agent/inbox/spliced` history — inheriting whatever input sat
UNCLAIMED in the parent at the click moment:

- next-turn: a queued follow-up behind a still-running turn;
- next-step (the reported long-context case, parent queue empty): a
  tool-result context / steering spliced between step boundaries.

The first side prompt then claimed [phantom, boundary, question] — the
stale parent message was sent to the model BEFORE the boundary and
logged as a user/message. The fix passes the exact marker pair the
host's own session.fork uses (api-session-controller); ownEvents() now
stops at the end-seed boundary and the inherited inbox replays empty.

Regression tests run the REAL Session.create + Inbox in both
directions (old shape inherits the phantom, marked shape stays empty);
routes tests pin the two create options. Verified: pnpm typecheck /
test (5 pre-existing unrelated UI-spec failures on main unchanged) /
build.

AGENTS.md

@@ -39,6 +39,7 @@
 8. **e2e scratch profile 的 `minimumReleaseAgeExclude` 含 `'@deepseek-ai/*'`**（`scripts/e2e-mount.sh` / `e2e-aggregate-mount.sh`，与仓库根 `pnpm-workspace.yaml` 同策）：alpha 版本常在发布后 24h 内跑 lane，pnpm 11 的 `minimumReleaseAge` 默认会拒装新鲜包。
 9. **插件开发树的 dsh-* 传递 peer 需提升为 devDependencies**（alpha.3 首见）：`dsh-subagent` 等 npm 包把 `dsh-attachment` 等 dsh-* 姊妹包全部声明为 peerDependencies（由宿主 bundle 树统一提供，宿主侧无此问题），插件仓库若只直接依赖其中一部分，其余 peer 在 pnpm 下会解析到树上残留的旧版——如 `dsh-attachment@0.1.1-rc.1` 缺 `admitPromptContent` 导出、`dsh-subagent` 产物 import 它时测试加载即崩。因此 devDependencies 需涵盖 dev 树实际触达的全部 peer（attachment / code-runtime / scope / session-projection / system-prompt / user-approval / util-time 七个即为此提升，与直接依赖同款精确钉版）；`@deepseek-ai/cordis` peer 自 alpha.3 起要求 `^4.0.2`（上游全线 peer 已升）。适配新 alpha 版本时先跑 `pnpm peers check`，把新失配的传递 peer 一并提升进 devDependencies。例外：**`dsh-client-locale` 的 peer/devDep 允许落后于基线**（alpha.5 时上游停在 0.1.2-alpha.3 未发新版）——peer 下限 `^0.1.2-alpha.3` 天然容纳同 tuple 的 alpha.5 运行时，此时保持旧钉版并在 `pnpm-workspace.yaml` 注明，待上游发版再追平；rc.1 起上游恢复发版，peer/devDep 已与基线一并钉 `0.1.2-rc.1`，该例外消除。
 10. **聊天文件打开漏斗是 `remote.session.openWorkspacePath`**（`ctx.workspaces.openPath` 已随 alpha 删除，`IWorkspaces` 无此方法）：「聊天区文件在侧边栏打开」拦截在 `ctx.inject(['remote.session'], …)` 内以 **defineProperty 数据属性遮蔽**该命名空间方法（gateway client 的方法是 accessor 属性、异步挂载、contribution 重挂载时服务重建——inject 回调重跑即自愈）；实现见 `src/client/openpath-intercept.ts`，设计见 [docs/plans/2026-08-31-openpath-intercept-alpha-design.md](docs/plans/2026-08-31-openpath-intercept-alpha-design.md)。
+11. **自定义种子必须带 fork 标记对（`meta.isSeeded: true` + `inheritedEventCount`）**：dsh-session 契约「只给 seed 不标 isSeeded，种子算重放历史而非继承前缀」——缺标记时 `Session.ownEvents()` 含整个种子，子会话 `Inbox` 构造器重放种子里的 `agent/inbox/spliced`，**继承父会话切割时刻未领取的 inbox 输入**（排队用户消息 next-turn、长回合中工具结果上下文/steering next-step——后者即「上下文很长时侧边对话先把之前的 User msg 发出去」的根因，幽灵消息排在 boundary 之前最先发给模型）。宿主 `session.fork`（api-session-controller）的调用即规范形态；回归由 `tests/sidechat-seed-validation.spec.ts`（真实 `Session.create` + `Inbox`）守护。0.1.2-rc.1 复评：无更优雅的 sidechat 宿主 API（`ContinuableStartSpec` 仍无 seed 字段），`AgentRegistry.create` 接缝补齐标记即为规范用法。
 
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