refactor(agent-loop): simplify message machine

packages/core/agent-loop/README.md

@@ -55,7 +55,7 @@ Configured agents start automatically. A model call requires both `provider` and
 
 The concrete `ReactLoopAgent`, its queued input, outbox, and run controls are package-internal. The package root exports only the plugin/service/config contract, and the package exports map exposes no `./src/*` escape hatch; lifecycle owners create agents through `ctx.agents` rather than naming, constructing, or starting driver internals. One prepared session can be claimed by only one concrete driver, and everything observable happens through session events and the `agent/*` event taxonomy.
 
-The unified `send()` primitive routes content and source by (`target` × `wakeup`); `followup`/`steer`/`inject` are its fixed-preset aliases. A `next-turn` item joins the queued FIFO, waking the driver unless `wakeup: false`; admission happens before any turn opens. The loop opens a private next-step acceptance window before `agent/prompt-submit` and closes it before `turn/end`. During that window, `steer()` and `inject()` stage in one outbox; an allowed admission opens the turn, records the prompt and returned `additionalContexts`, then drains the staged input before the first request. A blocked or failed admission writes no prompt or hook-produced context. A caller-staged context-only batch then takes idle injection's immediate append, while steering and context staged beside it remain pending for retry or a later admitted prompt. Outside the window, steering becomes a waking queued prompt and injection immediately appends `user/message` without opening a turn or running the model. Every inbox enqueue publishes `agent/inbox/enqueue` with the resolved queued-or-steering placement; taking it publishes `agent/inbox/dequeue` with the same placement, and `cancel()` without `keepInbox` publishes `agent/inbox/discard`.
+The concrete driver routes `followup()`/`steer()`/`inject()` through one private `send()` primitive. A follow-up joins the queued FIFO and wakes the driver; admission happens before any turn opens. The loop opens a private next-step acceptance window before `agent/prompt-submit` and closes it before `turn/end`. During that window, `steer()` and `inject()` stage in one outbox; an allowed admission opens the turn, records the prompt and returned `additionalContexts`, then drains the staged input before the first request. A blocked or failed admission writes no prompt or hook-produced context. A caller-staged context-only batch then takes idle injection's immediate append, while steering and context staged beside it remain pending for retry or a later admitted prompt. Outside the window, steering becomes a waking queued prompt and injection immediately appends `user/message` without opening a turn or running the model. Every inbox enqueue publishes `agent/inbox/enqueue` with the resolved queued-or-steering placement; taking it publishes `agent/inbox/dequeue` with the same placement, and `cancel()` without `keepInbox` publishes `agent/inbox/discard`.
 
 ### Loop lifecycle (`agent.ts`)
 
@@ -65,7 +65,7 @@ Every provider call that reaches a successful finish appends exactly one `assist
 
 After `agent/request` returns a provider/model call config, the loop asks `ctx.llm.prepareCall()` to validate any adapter-owned reasoning effort and materialize its configured default under the active turn signal. The prepared call retains the exact adapter registration across this asynchronous resolution, `request/header` logging, and terminal dispatch, so HMR cannot mix one adapter's capability result with another adapter's request. The effective config is logged before dispatch, so a listener can change effort between steps without hidden request drift. A route with no registered adapter preserves the proposed config so an `llm/stream` listener can own and short-circuit it; unhandled terminal dispatch still fails with `NO_ADAPTER`. A new loop instance restores the last effort only when its initial provider/model route exactly matches the logged route; a route change discards that opaque model-owned ID and resolves the new model independently.
 
-Plugin failure ends the current turn, not the loop. Only final adapter dispatch/iteration failures and terminal in-band error or aborted finishes enter `agent/request-error`; middleware, result processing, tools, and other extension failures close directly. Recovery receives the exact live error, immutable provider facts, immutable prior failures, the immutable retry policy of the adapter registration that served the request, and the turn signal after the failed step closes; the policy is absent if no final adapter served it. A handling listener returns `{ kind: 'retry' }`; the loop closes the failed turn with its error and opens one numbered retry turn without an intervening idle notification. Success clears the consecutive history, and an unhandled failure is terminal. AgentLoop owns one cancellation signal for the current admission or turn. An effective `cancel(cause)` clears pending work unless `keepInbox` is set and cooperatively aborts that signal; idle cancellation is a no-op. Durable `turn/end` records `aborted` for `user` and `parent`, while disposal records `disposed`; undispatched model tool calls receive synthetic `tool/call` and `ABORTED_BEFORE_DISPATCH` result pairs. The cancellation cause changes reporting, not how result context finalized after cancellation is handled. Disposal waits for signal-ignoring work before registry removal. The [explicit-cancellation decision](../../../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md) owns the lifecycle and race contract.
+Plugin failure ends the current turn, not the loop. Final adapter selection, dispatch, and iteration failures arrive from `ctx.llm` as terminal error or aborted finishes and enter `agent/request-error`; middleware, result processing, tools, and other extension failures remain thrown and close directly. Recovery receives request coordinates, immutable provider facts, the immutable retry policy captured by the prepared adapter registration, and the turn signal; the policy is absent when middleware owns an unprepared route. A handling listener returns `{ kind: 'retry' }`; an unhandled failure is terminal. AgentLoop owns one cancellation signal for the current admission or turn. An effective `cancel(cause)` clears pending work unless `keepInbox` is set and cooperatively aborts that signal; idle cancellation is a no-op. Durable `turn/end` records `aborted` for `user` and `parent`, while disposal records `disposed`; undispatched model tool calls receive synthetic `tool/call` and `ABORTED_BEFORE_DISPATCH` result pairs. The cancellation cause changes reporting, not how result context finalized after cancellation is handled. Disposal waits for signal-ignoring work before registry removal. The [explicit-cancellation decision](../../../.agents/notes/implemented/architecture/2026-07-16-explicit-turn-cancellation.md) owns the lifecycle and race contract.
 
 Within a step, exclusive calls form barriers; parallel-safe calls use a bounded rolling pool and are reclassified before start. Only dispatch/body overlaps. Policy, durable results, and result context remain model-ordered. Abort stops new calls, drains started results, and retains their finalized result context without distinguishing the cancellation cause.