fix(apiproxy): echo the preset a created session runs, not its header

`session.create` also adopts an already-live session, and the preceding
commit newly allows adopting one under the preset it switched to while
blank. Its response still echoed `header.agentPreset`, so that adoption
answered with the preset the session had just left — contradicting the
request it had accepted and the row `session.list` serves for the same
session from `resolveSessionPreset()`. The echo now resolves the same way.

The `assertPresetUnchanged` parameter doc said `existing` was the preset
the session was created under; both callers now pass what it runs.

`composeFrom()` was documented as "infallible" and "cannot fail" beside two
`@throws`. It has no composition failure mode — no roster read, no mount, no
file — but it does reject a caller error, and the wording now says which.

The package-level "switched preset" test re-linked to the same preset id,
so it could not tell reading the parent's live scope chain from reading its
creation header. A second fixture preset makes the switch real.

The Web browser lane's subagent goldens gain the preset badge a child now
shows, which is the visible consequence of recording its composition. That
lane runs only under DSH_EXAMPLE_MODE=lib and was missed before.

The Agent Note records two limits found in review: a cold-resumed
continuable child joins its parent's current composition rather than the one
its header names, and `toolFilter` does not constrain a joined child. The
latter is a regression from the agent-plane move rather than anything this
change introduces — with the same tools in the global layer the filter
applies normally — and is tracked in #2185.

Refs #2185

.agents/notes/implemented/bug-fix/2026-08-10-child-agents-join-their-parent-preset.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-08-10-child-agents-join-their-parent-preset.md
-2026-08-10-child-agents-join-their-parent-preset.md: c9917c48d10c2b2515284405ea52aed8b476f8b1
-2026-08-10-child-agents-join-their-parent-preset.zh.md: 09e4de5292b65e50bb3973704fd803c819be9f1c
+2026-08-10-child-agents-join-their-parent-preset.md: d9aa0dc43c1338d3198f5335da6ad238730d58a1
+2026-08-10-child-agents-join-their-parent-preset.zh.md: dd85c642ff7e6e2934e805c2efaccdc6dda63f15

.agents/notes/implemented/bug-fix/2026-08-10-child-agents-join-their-parent-preset.md

@@ -38,10 +38,16 @@ This is a bind, not a mount, and both differences are load-bearing. The child ge
 
 `packages/preset/agent-presets/tests/mount.spec.ts` covers the join against real fixture compositions: the child sees its parent's tools and prompt sections, no second generation is mounted, the join survives the parent's disposal (a background child outliving its parent), the reported id matches, a parent without a preset joins nothing, and an unscoped context is refused.
 
-`packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts` asserts the model-visible result through `startInProcessRun()` on a host composition carrying no model-facing rows: the schemas in the child's own request, its parent's prompt section, the recorded header preset, and a parent that switched preset while blank.
+`packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts` asserts the model-visible result through `startInProcessRun()` on a host composition carrying no model-facing rows: the schemas in the child's own request, its parent's prompt section, the recorded header preset, and a parent that switched preset while blank — to a DIFFERENT preset, so the assertion distinguishes reading the parent's live scope chain from reading its creation header.
+
+The assembled-transcript layer is the shipped Web composition's e2e rather than a keyless snapshot. Every runnable example this repo ships composes no preset roster, so the defect is not observable in the snapshot harness at all: a snapshot scenario would first need an example that mounts a roster AND delegates. The Web e2e boots the real `base` + `web-app` patch layers with both shipped presets, which is the assembled evidence the testing policy asks for; the Web browser lane's subagent goldens carry the visible consequence, since a child that records its preset now shows the preset badge its parent shows.
 
 ## Consequences
 
-Delegation now costs a scope-parent bind per child and nothing else — no extra plugin instances, no roster read, no failure mode. A child's capabilities are exactly its parent's, minus whatever its own `toolFilter` removes; a per-subagent preset ("agent types") remains unbuilt and would be a new request field rather than a change to this join.
+Delegation now costs a scope-parent bind per child and nothing else — no extra plugin instances, no roster read, no failure mode. A child's capabilities are exactly its parent's — the per-child `toolFilter` does not narrow them, for the separately tracked reason below; a per-subagent preset ("agent types") remains unbuilt and would be a new request field rather than a change to this join.
 
 `applyChildComposition()` changed shape, so any future out-of-tree in-process driver must supply the parent. That is the intended cost: the previous signature let a caller compose a capability-less child and get no error.
+
+A cold-resumed continuable child joins its parent's CURRENT composition rather than the one its own header records. The window is narrow — the parent must create the child, stay blank, switch preset, and only then wake it, since a resident child never re-joins and a one-shot child never resumes — and the alternative is worse: resolving the child's own recorded id would re-read the roster and hand back the preset-deleted failure mode this join exists to avoid. The child's header still records what it started under, so the divergence is observable rather than silent.
+
+`toolFilter` does not constrain a joined child, because `ToolRegistry` compiles restrictions against global-layer names only and overlays chain-layer tools unfiltered. That is not new here — with the roster composed, `tools.restrict()` already rejected every name as an unknown global tool, so a child carrying a filter failed to start both before and after this change — but it is a regression from the agent-plane move rather than a standing limitation: with the same tools registered in the global layer, the filter admits and applies normally. It matters more now that the child has its parent's full tool set to be restricted from. It is tracked separately; this change neither introduces nor repairs it.

.agents/notes/implemented/bug-fix/2026-08-10-child-agents-join-their-parent-preset.zh.md

@@ -38,10 +38,16 @@ Status: implemented
 
 `packages/preset/agent-presets/tests/mount.spec.ts` 用真实 fixture 组装覆盖该加入：子 agent 看到父方的工具与提示段、不会挂载出第二个代际、加入在父方 dispose 后依然成立（活得比父方久的后台子 agent）、上报的 id 一致、没有 preset 的父方不产生加入、以及无 scope 的上下文被拒绝。
 
-`packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts` 在一个不含任何面向模型行的宿主组装上，通过 `startInProcessRun()` 断言模型可见的结果：子 agent 自身请求中的 schema、父方的提示段、记录下来的 header preset，以及在空白期切换过 preset 的父方。
+`packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts` 在一个不含任何面向模型行的宿主组装上，通过 `startInProcessRun()` 断言模型可见的结果：子 agent 自身请求中的 schema、父方的提示段、记录下来的 header preset，以及在空白期切换过 preset 的父方——切换到**另一个** preset，这样断言才能区分"读父方活 scope 链"与"读父方创建 header"。
+
+组装记录这一层用的是真实 shipped Web 组装的 e2e，而不是无密钥快照。本仓库所有可运行 example 都不组装 preset roster，因此该缺陷在快照 harness 里根本不可观察：要做快照场景，得先有一个既挂载 roster 又发起委派的 example。Web e2e 启动的是真实的 `base` + `web-app` 补丁层与两个 shipped preset，这正是测试政策要求的组装证据；Web 浏览器 lane 的 subagent golden 承载了可见后果——记录了 preset 的子 agent 现在会显示与其父方相同的 preset 徽标。
 
 ## Consequences
 
-委派现在的成本是每个子 agent 一次 scope 认父，再无其他——没有额外的插件实例、没有 roster 读取、没有新的失败模式。子 agent 的能力恰好等于父方的能力，减去它自己的 `toolFilter` 所移除的部分；逐 subagent 的 preset（"agent 类型"）仍未构建，那会是一个新的请求字段，而不是对这次加入的改动。
+委派现在的成本是每个子 agent 一次 scope 认父，再无其他——没有额外的插件实例、没有 roster 读取、没有新的失败模式。子 agent 的能力恰好等于父方的能力——逐子 agent 的 `toolFilter` 并不能收窄它，原因见下方另行跟踪的那条；逐 subagent 的 preset（"agent 类型"）仍未构建，那会是一个新的请求字段，而不是对这次加入的改动。
 
 `applyChildComposition()` 的形态变了，因此将来任何仓库外的进程内驱动都必须提供父方。这是刻意付出的代价：此前的签名允许调用方组装出一个毫无能力的子 agent 而不报任何错。
+
+冷恢复的可继续子 agent 加入的是父方**当前**的组装，而不是它自己 header 所记录的那份。窗口很窄——父方必须先建子、保持空白、切换 preset，之后才唤醒它；驻留中的子 agent 不会重新加入，一次性子 agent 也不会恢复——而替代方案更糟：按子 agent 自己记录的 id 解析会重读 roster，把这次认父刻意规避掉的"preset 已删除"失败模式又请回来。子 agent 的 header 仍记录它启动时的那份，因此这处分歧是可观察的而非静默的。
+
+`toolFilter` 约束不住已加入组装的子 agent，因为 `ToolRegistry` 只按全局层的名字编译限制，随后把 scope 链上的工具无过滤地叠加进来。这不是本次改动带来的——在组装了 roster 的部署里，`tools.restrict()` 本就把每个名字都判为未知全局工具，因此带过滤器的子 agent 在本次改动前后同样起不来——但它是搬到 agent 平面所引入的回归，而非长期存在的限制：同样这批工具注册在全局层时，过滤器能正常校验并生效。现在子 agent 有了父方的全套工具需要被限制，它变得更要紧。该问题另行跟踪；本次改动既未引入也未修复它。