feat(ui): make a session that cannot send refuse to accept one

A default naming a route the Models page has since removed left the
composer saying 选择模型 while the input still accepted a message, which
then failed inside the adapter mid-turn.

`session.prompt` now refuses with `model-unavailable` before opening a
turn. That is the enforcement boundary: the method stays callable no
matter what a client disables. `session.models` reports the same fact as
`routable`, and ui-model pushes a block through the new
`ctx.conversation.blocks` registry so the bar renders the disabled
textarea it already renders without a workspace, carrying the blocker's
own reason. The push direction is forced — ui-model already depends on
ui-conversation, so ui-conversation cannot read it back.

The gate is `routable`, not "matches no advertised group": catalog
membership is advisory, so a route serving a model it stopped advertising
is missing from the groups yet perfectly usable, and `null` before the
first load never blocks so a slow Host cannot lock a working composer.

The scaffold gains a route-only adapter for fixture-less keyless
scenarios. Registering zero providers is a test artifact — every product
composition mounts one — and the goldens that froze the seat's fallback
label now show the model those scenarios actually route to.

packages/host/apiproxy/README.md

@@ -10,7 +10,9 @@ The API gateway every client shape shares: the TS contract (`src/api/`, zero Nod
 
 A session resolves its route from three tiers, re-read on every access rather than seeded once: a selection made in this process, else the session's own latest logged `request/header`, else this default. Re-reading is what makes both directions hold — a session that has run a turn derives its route from its log forever after, so changing the default never retargets it, while a session still blank (New Session reuses one rather than minting another) starts from a default saved after it was created.
 
-`session.selectModel` records an accepted switch as the new default, which is how the default is chosen in practice: there is no separate gesture. The write replaces the section wholesale rather than merging, because switching to a model with no reasoning effort has to clear a stored one; a storage failure is logged without undoing the switch, which already applies to its own session. A deployment with no settings provider keeps the composition entry and a switch stays process-local.
+`session.selectModel` records an accepted switch as the new default, which is how the default is chosen in practice: there is no separate gesture. What it stores is the RESOLVED target, so an adapter-materialized default effort is pinned as the user saw it and a later adapter-default change does not silently move stored defaults. The write replaces the section wholesale rather than merging, because switching to a model with no reasoning effort has to clear a stored one; a storage failure is logged without undoing the switch, which already applies to its own session. A deployment with no settings provider keeps the composition entry and a switch stays process-local.
+
+The section's `reasoningEffort` has no counterpart in the plugin config, deliberately: the seam merges the user layer over the composition entry per field, so an absent key cannot override a present one and a composition-set effort would survive every later switch to a model without one. A deployment default for effort belongs on the adapter profile, which resolves per model.
 
 The stored route is not validated against the registry, in either direction. A default naming a route the Models page has since removed still reaches `session.models` as the session's `current` — matching no advertised group, which is precisely what makes a selector prompt for a replacement instead of naming a model the deployment cannot reach. Repairing it silently would also break the deliberate converse: an adapter may serve a model its catalog does not advertise.
 
@@ -30,7 +32,7 @@ Session titles ride the generic projection pair like every other domain — the
 
 `session.fork` maps an optional event anchor to the first `turn/end` at or after it, letting a message action include that message's whole turn. An omitted or past-end anchor selects the last completed turn; an in-log anchor whose turn remains open returns `fork-unavailable` rather than clipping backward. The published child inherits the source's seeded history, cwd, latest logged provider/model/reasoning target, and lineage before joining the source Workspace. If Workspace attachment fails, `workspace-attach-failed` carries the already-published child id so clients can reconcile it. The [SessionStore fork decision](../../../.agents/notes/implemented/feature/2026-06-30-session-store-fork-api.md) owns the boundary rationale.
 
-Session model routing is a session-domain contract. `session.models` returns the selected provider/model/reasoning target separately from provider-grouped advisory models, exact-route reasoning metadata, and provider-local lookup failures. The current target may be absent from the groups and is never injected as a synthetic row; clients can prompt for a replacement without turning the directory into a routing whitelist. `session.selectModel` validates the optional adapter-owned reasoning effort and replaces the complete target selected for the next prompt-assembly boundary. Catalog membership is not validation: an adapter may resolve an unlisted model, while an unavailable route or unsupported effort returns `model-unavailable`.
+Session model routing is a session-domain contract. `session.models` returns the selected provider/model/reasoning target separately from provider-grouped advisory models, exact-route reasoning metadata, and provider-local lookup failures. The current target may be absent from the groups and is never injected as a synthetic row; clients can prompt for a replacement without turning the directory into a routing whitelist. `session.selectModel` validates the optional adapter-owned reasoning effort and replaces the complete target selected for the next prompt-assembly boundary. Catalog membership is not validation: an adapter may resolve an unlisted model, while an unavailable route or unsupported effort returns `model-unavailable`. `session.models` additionally reports `routable`: whether an adapter currently serves the current target's route, which is deliberately NOT derivable from the groups — a route serving a model it stopped advertising is absent from them yet perfectly usable, while a route whose adapter is gone can serve nothing. `session.prompt` refuses on that same fact with `model-unavailable` rather than spending the pre-step path to fail inside an adapter; a client that disables its composer is an affordance, and this method stays callable regardless.
 
 Pending queued input is a live control-plane contract, not conversation history. The gateway derives the complete `next-turn` queue from durable `agent/inbox/spliced` mutations and broadcasts authoritative `session/queue` snapshots after each change and on reconnect; pending `next-step` steering stays outside this Web projection. Within `next-step`, user-origin messages carry the `steering` placement while injected context (approval notices, task completion, attached snapshots) carries `context` and is not surfaced until claimed. The message-local `agent/inbox/inserted`, `claimed`, and `discarded` notifications remain available to lifecycle observers but do not build the queue view. `session.updateQueue` addresses one `MessageId`; edit and remove mutate the attached Agent through `Inbox.splice()`. A claim's pure deletion splice wins races before pre-step admission, so a later operation returns `queue-item-not-found`. `session.cancel` aborts only the active turn and preserves pending inbox work; after cancellation reaches quiescence and the closing turn flushes, AgentLoop claims the next waking message in FIFO order, and the browser never resends or promotes it. Queue operations never resume a cold session, and the client never infers retirement from turn or status events.