fix(preset): keep the token meter host-plane and name unjoined agents

Moving the agent plane behind presets left two readers on the wrong side of
the host/agent line.

`dsh-token-meter` was disabled on the host and mounted inside each preset's
`compaction` realm, but its three projection units register into the
process-wide `sessionProjections` table. A unit registered from one preset
answers for every session, so whether a `minimal` session showed a context
meter depended on whether some other session had mounted `standard` since
boot, and a process that only ever ran `minimal` showed none. The meter takes
no configuration, keys every fold by Session, and registers no tool or prompt
section, so it returns to the host composition and leaves the presets'
`isolate` map; the realm and `compact-basic` stay, because what a preset
chooses is whether its agent compacts, not whether its tokens are counted.

Nothing named an agent that joined no preset. The join is a scope-parent link,
and without it the tools, prompt-section, and skill views resolve the empty
global layer: the agent publishes, the turn runs, and the model receives
nothing. `AgentPresets` now logs one warning per such agent while a roster is
configured, and the invariant companion fails outright — at
`system-prompt/assemble` rather than at publication, because an unjoined agent
is legal until it addresses a model and `recompose` binds exactly such an
agent. The warning stays advisory: a synchronous `agent/created` throw vetoes
publication, and the ACP bridge, SDK server, and headless bundle all create an
unjoined agent today.

Three limits are recorded rather than fixed: projection key presence is not a
per-session capability signal, a superseded standing generation is never
reclaimed, and a `cordis_mount` temporary plugin belongs to the composition
rather than the session that mounted it.

Fixes #2203

packages/session/session-projection/README.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 packages/session/session-projection/README.md
-README.md: 2615b253999c798172168ec9d0232eb965b07fbc
-README.zh.md: 0712e9c7a61fbcf43939791b3e7cd24af6777c3a
+README.md: 9018b133bb69ed4717fede14c9a2070a07c3fa62
+README.zh.md: b91908117fd452855a82976515d13165803def43

packages/session/session-projection/README.md

@@ -42,6 +42,7 @@ None; projections never assemble or send provider requests.
 ## Known Limitations and Deferred Work
 
 - **Every tail page carries every registered key** — there is no per-key opt-out or lazy-key request shape yet; acceptable while values are UI-scale whole states (a todo list, a goal snapshot), revisit if a domain's value grows large.
+- **The unit table is process-wide, so key presence is not a per-session capability signal** — a key registered by ANY agent preset appears in every session's snapshot, including sessions whose own composition mounts nothing that produces it. A client must read the VALUE (`plan.active`, an empty todo list) rather than treat an absent key as absence of the feature; a unit whose empty value is indistinguishable from a real one belongs on the host plane instead, which is why `dsh-token-meter` sits there.
 - **Eager drive touches every unit per event** — cheap by construction (whole-value rule, same-reference gate), but a hot path would justify per-unit event-type prefilters, addable without contract change.
 - **Registry cells live in memory only** — a restart rebuilds by folding the log on first touch; compositions that mount `dsh-session-projection-cache` seed that fold from persisted rows instead.
 - **Synchronous unit discipline is only partially mechanical** — the boundary `schema.parse` rejects a Promise-returning `view`, but an `apply` that blocks or reads torn non-session state is a review concern; the invariant companion documents why no runtime check exists.

packages/session/session-projection/README.zh.md

@@ -42,6 +42,7 @@
 ## 已知限制与暂缓事项
 
 - **每个尾页携带每个已注册的 key**——尚无逐 key 的 opt-out 或惰性 key 请求形状；在值都是 UI 量级的全量状态（一张 todo 清单、一份 goal 快照）时可以接受，若某领域的值变大再重议。
+- **单元表是进程级的，因此 key 是否存在不能当作逐会话的能力信号**——只要**任何**一个 agent preset 注册了某个 key，它就出现在每个会话的快照里，包括自身组装完全不产出该值的会话。客户端必须读**值**（`plan.active`、空的 todo 列表），不能把 key 缺席当作功能缺席；如果某个单元的空值与真实值无法区分，它就该待在宿主平面——`dsh-token-meter` 正因如此留在那里。
 - **主动驱动（eager drive）逐事件触达每个单元**——按构造开销很低（全量值规则、同引用闸门），但若出现热点路径，可加按单元的事件类型预过滤，约定不变。
 - **注册表 cell 只活在内存里**——重启后首次触达时靠折叠日志重建；挂载了 `dsh-session-projection-cache` 的组合改由持久行播种该折叠。
 - **单元同步纪律只有部分可机械把关**——边界 `schema.parse` 能拒绝返回 Promise 的 `view`，但阻塞的 `apply`、或读取撕裂的非会话状态的 `apply`，只能靠评审把关；invariant 配套记载了为何不存在运行时检查。