docs(agent-presets): record what a live agent costs, and that nothing evicts one

Review asked whether composing a preset per session compounds memory.
Measured against the shipped compositions: it does not compound — growth
is strictly linear at ~0.17 MB per agent on `minimal` and ~1.31 MB on
`standard`, the first agent of a process pays ~7 MB more for module
imports every later mount shares, and disposal returns essentially all
of it.

What the measurement did find is that nothing disposes. The api-proxy
discards the handle it creates, archiving only edits the workspace
registry, the registry has no eviction, and the sole disposal site in
the host is the JSON-RPC server's shutdown. A preset did not introduce
that; it raised the per-session price from ~0.2 MB to ~1.3 MB and made
it visible.

Recorded as a remaining TODO rather than fixed here: eviction belongs to
the host that owns the handle, not to this seam.

.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: c66503bf47b3b571c6c31673b2d975571a3af93d
-2026-08-03-per-session-agent-presets.zh.md: 21f021c6bbaa11c33bc93104aba0f6e8ce2f1448
+2026-08-03-per-session-agent-presets.md: 51f1d6790eb763e6ad2f534fb2e1137ecbcba1f2
+2026-08-03-per-session-agent-presets.zh.md: c1881c372880e56adc74a46e20c5ec9bd34b36aa

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

@@ -67,6 +67,10 @@ Which preset an unnamed session gets is a user setting (`agent-presets.default`)
 
 **The choice belongs to the screen where it still works.** The composer seat spent almost its whole life disabled, since the preset is fixed once a turn has run. It moved to the new-session screen beside the workspace picker, where the pick is *staged*: that screen precedes the session it applies to, and the stage lands when a session becomes current and is still blank — covering both the session a workspace connect creates and the blank one it reuses, which riding `sessions.create` would miss. It is spent on first use, matching the workspace picker beside it. What a running session runs is then a read-only label in its header: a control there would promise a switch the host refuses outright.
 
+**A preset multiplies a cost the host was already paying: nothing disposes an agent.** Measured against the shipped compositions with `--expose-gc`, one live agent holds ~0.17 MB on `minimal` and ~1.31 MB on `standard`/`cordis`, mounting in ~38 ms and ~135 ms; the first agent of a process costs ~7 MB more as Node imports the modules, which every later mount then shares. Growth is strictly linear — 10, 30 and 50 agents give the same per-agent delta — and disposal reclaims essentially all of it (50 `standard` agents held 57.8 MB and returned it). So the object graph does not leak; the lifecycle does. `dsh-host-apiproxy` discards the `AgentHandle` it creates, `archiveSession` only edits the workspace registry, `AgentRegistry` has no eviction, and the sole disposal site in the host is the JSON-RPC server's own shutdown. A web host therefore retains every session it has touched, at ~1.3 MB each once presets are composed rather than ~0.2 MB before. Note that pruning the mount registry does not help here: it drops records whose fiber `uid` has cleared, and an agent that never dies never clears one.
+
+- Remaining TODO: idle agent eviction — dispose after the session is persisted and re-mount on resume. It belongs to the host that owns the handle, not to this seam.
+
 ## Alternatives considered
 
 **Add a preset tier to the scoped registries.** `ScopedLayers.merge()` combines the global layer with exactly one exact-scope layer. A middle tier would let many sessions share one mounted composition, but it changes `dsh-scope` and every scope-aware registry to save a cost measured in milliseconds, and it gives a preset's registrations a lifetime no agent owns.

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

@@ -68,6 +68,10 @@ Status: implemented
 
 **这个选择属于它仍然可用的那个界面。** composer 座位几乎一生都处于禁用状态，因为一旦跑过一个轮次，preset 即固定。它移到了新建会话界面、工作区选择器旁边，选择在那里是**暂存**的：该界面先于它要应用到的会话存在，暂存值在某个会话成为当前会话且仍为空白时落地——这既覆盖工作区连接新建的会话，也覆盖它复用的那个空白会话，而搭 `sessions.create` 的便车会漏掉后者。它一经使用即被清空，与旁边的工作区选择器一致。至于运行中的会话在跑什么，则是其标题旁的一个只读标签：在那里放控件，等于承诺一次宿主会断然拒绝的切换。
 
+**preset 放大的是宿主本来就在付的代价：没有任何东西会 dispose 一个 agent。** 用 `--expose-gc` 对随附组装实测：一个存活的 agent 在 `minimal` 上约占 0.17 MB、在 `standard`/`cordis` 上约 1.31 MB，挂载耗时分别约 38 ms 与 135 ms；进程里第一个 agent 另需约 7 MB，那是 Node 首次 import 模块的一次性成本，此后每次挂载共享。增长严格线性——10、30、50 个的单个增量一致——且 dispose 后基本全额回收（50 个 `standard` 占住 57.8 MB，释放后全部归还）。所以对象图并不泄漏，缺的是生命周期。`dsh-host-apiproxy` 创建后直接丢弃 `AgentHandle`，`archiveSession` 只改工作区注册表，`AgentRegistry` 没有驱逐机制，而宿主里唯一一处 dispose 是 JSON-RPC 服务器自身的关停。于是一个 web 宿主会留住它接触过的每一个会话，组装 preset 之后每个约 1.3 MB，而在此之前约 0.2 MB。注意：剪枝挂载注册表在这里没有用——它丢弃的是 fiber `uid` 已清空的记录，而永不死亡的 agent 永远不会清空它。
+
+- 遗留 TODO：idle agent 驱逐——会话持久化后 dispose，恢复时重新挂载。它属于持有 handle 的那个宿主，不属于本 seam。
+
 ## 考虑过的替代方案
 
 **在 scope 注册表中新增 preset 分层。** `ScopedLayers.merge()` 把全局层与恰好一个精确 scope 层合并。新增中间层可以让多个会话共用一份已挂载的组装，但它要改动 `dsh-scope` 及每个 scope 感知的注册表，换来的只是毫秒级的开销节省，而且会让 preset 的注册获得一个没有任何 agent 拥有的生命周期。