fix(subagent): compose children from their parent's preset

Tool and prompt-section visibility is inherited along dsh-scope's parent
chain, and an agent's scope key is minted with no parent. Per-session agent
presets moved every model-facing row onto the agent plane and made
AgentPresets.mount() the one thing that binds that link, from the api-proxy's
session create, resume, and fork paths. The two in-process subagent drivers
installed only the per-child persona and tool filter, so a child's scope chain
had length one and its registry view resolved the global layer alone — which
is empty wherever a preset roster is composed. One-shot children reached the
model with no tools, continuable ones with only the host-plane `report`, and
neither carried its parent's persona, workspace context, or skill catalog.

AgentPresets.composeFrom() joins one agent to the standing composition another
already runs on. It is a bind, not a mount: the child gets its parent's exact
generation, so a composition edited since the parent started cannot fork it
onto another one, and it is synchronous, which is what lets a child creation
window use it. applyChildComposition() now takes the parent and performs the
join first, making a child composed without it unrepresentable at the call
sites. childSessionMeta() records the joined id so a cold read rebuilds the
composition the child actually ran under.

The audit that followed found two api-proxy readers on the wrong authority:
presenterScopeFor() and the live-agent branch of assertPresetUnchanged() both
read header.agentPreset, which goes stale the moment a blank session switches
preset. A switched session's cold transcript resolved presenters in the older
composition's layer and silently degraded to generic cards, and the gateway
refused to adopt a live session under the preset it actually runs while
accepting the one it left. Both now resolve through resolveSessionPreset(),
matching the resume branch fifteen lines above. The owning architecture Agent
Note carried the stale claim that the header records what a session runs; it
is corrected to name the header/log pair and its three readers.

Fixes #2165

.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.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/architecture/2026-08-03-per-session-agent-presets.md
-2026-08-03-per-session-agent-presets.md: 6f1643c25008c3363cb10adb7fbff7afeea31cbe
-2026-08-03-per-session-agent-presets.zh.md: 7afe9ade5c98fadb96384a7e0acd47531c370e0c
+2026-08-03-per-session-agent-presets.md: c39117ab0de001650a95f98ccf3e42f3a5034c92
+2026-08-03-per-session-agent-presets.zh.md: 5e98a2865013a355c134317ded8a4f2ddaccf42c

.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md

@@ -31,7 +31,7 @@ Which preset an unnamed session gets is a user setting (`agent-presets.default`)
 
 ## Consequences
 
-**The effective default is read per resolution, never snapshotted.** A cached value would need a `watch` subscription and a reload path to stay honest, and the resolved scope already re-reads a hot-reloaded document. Reading through is also what makes the boundary correct rather than merely cheap: the new value applies to the next session created, and every running session keeps the composition it was built from. That invariant is the same one the session header enforces from the other side — the header records the id a session actually runs, so a resume rebuilds that composition rather than today's default, and the gateway rejects an attempt to adopt a live session under a different one. A snapshot would make the two disagree at exactly the moment the setting changes.
+**The effective default is read per resolution, never snapshotted.** A cached value would need a `watch` subscription and a reload path to stay honest, and the resolved scope already re-reads a hot-reloaded document. Reading through is also what makes the boundary correct rather than merely cheap: the new value applies to the next session created, and every running session keeps the composition it was built from. That invariant is the same one the session log enforces from the other side — the header records the id a session was CREATED with and an `agent-preset/selected` event records any later blank-session switch, so a reader resolves the pair (`resolveSessionPreset`) and never the header alone: a resume rebuilds the composition its history was produced under rather than today's default, a cold transcript's presenters resolve in that composition's layer, and the gateway rejects an attempt to adopt a live session under a preset other than the one it currently runs. A snapshot would make the two disagree at exactly the moment the setting changes.
 
 **A directly-plugged subtree is invisible to the boot audit.** It never links itself to an `Entry`, so it is absent from `ctx.loader.entries()` and `assertEntriesActivated` cannot see it. The mount audits its own rows instead, reading the tree through an `Include` subclass that publishes it.
 

.agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.zh.md

@@ -31,7 +31,7 @@ Status: implemented
 
 ## 后果
 
-**有效默认值在每次解析时读取，从不快照。** 缓存下来就需要一个 `watch` 订阅和一条重载路径才能保持诚实，而解析后的 scope 本来就会重读热重载过的文档。读穿也不只是省事，它让边界本身是对的：新值作用于**下一个新建的会话**，每个运行中的会话保持它被构建时的那份组装。这条不变量正是 session header 从另一侧执行的同一条——header 记录会话实际运行的 id，因此恢复重建的是那份组装而不是当下的默认值，网关也会拒绝把一个活着的会话收编到另一个 preset 之下。快照会让两者恰好在设置改变的那一刻各说各话。
+**有效默认值在每次解析时读取，从不快照。** 缓存下来就需要一个 `watch` 订阅和一条重载路径才能保持诚实，而解析后的 scope 本来就会重读热重载过的文档。读穿也不只是省事，它让边界本身是对的：新值作用于**下一个新建的会话**，每个运行中的会话保持它被构建时的那份组装。这条不变量正是 session 日志从另一侧执行的同一条——header 记录会话**创建时**的 id，此后空白期的任何切换由 `agent-preset/selected` 事件记录，因此读取方解析的是两者之和（`resolveSessionPreset`）、绝不单看 header：恢复重建的是其历史所产出的那份组装而不是当下的默认值，冷读记录的 presenter 在那份组装的层里解析，网关也会拒绝把一个活着的会话收编到它当前运行的 preset 以外的 preset 之下。快照会让两者恰好在设置改变的那一刻各说各话。
 
 
 **直接挂载的子树对启动审计不可见。** 它不会把自己关联到 `Entry`，因此不在 `ctx.loader.entries()` 中，`assertEntriesActivated` 也看不到它。改由挂载过程自行校验各行，通过一个会公开自身 tree 的 `Include` 子类读取。