feat(agent): unify send(target × wakeup), coalesce context/message into user/message

Replace send/steer/inject with one Agent.send primitive over the
(target × wakeup) matrix; followup/steer/inject become fixed-preset
alias methods on the now-abstract Agent class. Coalesce context/message
into user/message (injected context is a non-user source). Replace
agent/queued with agent/inbox/enqueue/dequeue/discard, add cancel
keepInbox, and add a FIFO-conservation invariant.

packages/core/agent-loop/README.md

@@ -52,7 +52,7 @@ Configured agents start automatically. A model call requires both `provider` and
 
 The concrete `Agent` class, its `Inbox`, `runLoop`, and instance-bound publication/start 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.
 
-Each concrete `send()` materializes content, resolved source, and attached contexts once as a detached, deeply frozen lossless-JSON FIFO item. If claimed, it is the sole ordinary message in its turn; its contexts are the prompt waterfall's default additional contexts and therefore materialize only after admission. Absent or `separate` placement appends an independent `context/message`; `prompt-prefix` placement bakes the context, the stable `## My request:` delimiter, and effective request into one `user/message`, whose model-hidden envelope retains display content and context descriptors. The waterfall's returned allow is authoritative, so a listener wrapping `next()` preserves downstream `content` and `additionalContexts` unless it intentionally replaces them. A successor waits for the preceding ordinary turn's checkpoint to settle, while cancellation, disposal, a prompt block, or a pre-start failure may drop its contexts with the message. Running `steer()` enters the same record shape in the steering FIFO without dispatching `agent/prompt-submit`; its next checkpoint applies the same separate-or-prefix placement to `steering/message`, while policy can still stop before another step. Steering left after turn close and its checkpoint becomes later queued input with contexts intact unless terminal turn policy, cancellation, or disposal discards it. Open-turn `inject()` uses the same accepted-value boundary but defers in a FIFO while the current step executes assistant tool calls; successful batches place it after all results, and interrupted batches drain it before turn close. Malformed data throws before enqueue or append.
+The unified `send()` primitive materializes content, resolved source, and attached contexts once as a detached, deeply frozen lossless-JSON record, then routes it 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`); if claimed, it is the sole ordinary message in its turn, and its contexts are the prompt waterfall's default additional contexts that materialize only after admission. Absent or `separate` placement appends an independent injected `user/message`; `prompt-prefix` placement bakes the context, the stable `## My request:` delimiter, and effective request into one `user/message`, whose model-hidden envelope retains display content and context descriptors. The waterfall's returned allow is authoritative, so a listener wrapping `next()` preserves downstream `content` and `additionalContexts` unless it intentionally replaces them. A successor waits for the preceding ordinary turn's checkpoint to settle, while cancellation, disposal, a prompt block, or a pre-start failure may drop its contexts with the message. A running `next-step`/wakeup `steer()` enters the same record shape in the steering FIFO without dispatching `agent/prompt-submit`; its next checkpoint applies the same separate-or-prefix placement to `steering/message`, while policy can still stop before another step. Steering left after turn close and its checkpoint becomes later queued input with contexts intact unless terminal turn policy, cancellation, or disposal discards it. `next-step`/no-wakeup `inject()` bypasses the FIFOs and appends durable context directly: an open-turn injection uses the same accepted-value boundary but defers in a FIFO while the current step executes assistant tool calls (successful batches place it after all results, interrupted batches drain it before turn close), and an idle injection wraps a one-shot `injection` turn. Every FIFO enqueue publishes `agent/inbox/enqueue`; the driver's claims publish `agent/inbox/dequeue`, and `cancel()` without `keepInbox` publishes `agent/inbox/discard`. Malformed data throws before enqueue or append.
 
 ### Loop lifecycle (`loop.ts`)