feat(message-queue): per-conversation queue + steering injection

A per-conversation message queue (new message-queue extension) holds user
messages enqueued while a turn generates; delivered mid-turn as steering at the
tool-result boundary (or carried to a new turn if no tool call fires).

- kernel: RunTurnInput.drainSteering callback (generic; kernel stays pure)
- wire 0.7.0->0.8.0: QueuedMessage, QueuePayload, TurnSteeringEvent (additive)
- transport-contract 0.11.0->0.12.0: POST /conversations/:id/queue + chat.queue WS op
- message-queue ext: queue state + per-conversation custom surface (rendererId message-queue)
- session-orchestrator: enqueue facade + drainSteering wiring + post-seal carry
- transport-http/ws: queue endpoint + chat.queue op (fixes WsClientMessage exhaustive switch)
- host-bin: register message-queue

1043 vitest + 199 transport bun pass; tsc/biome clean; boot smoke clean.
FE courier: frontend-message-queue-handoff.md.

1 files changed, 17 insertions, 0 deletions

diff --git a/packages/kernel/src/contracts/runtime.ts b/packages/kernel/src/contracts/runtime.ts
index b7fe23c..c449a68 100644
--- a/packages/kernel/src/contracts/runtime.ts
+++ b/packages/kernel/src/contracts/runtime.ts
@@ -100,6 +100,23 @@ export interface RunTurnInput {
 	 * absent) — backward-compatible with callers that don't provide a clock.
 	 */
 	readonly now?: () => number;
+
+	/**
+	 * Optional. Called by the runtime at the tool-result boundary — after a
+	 * step whose tool calls have all executed, before the next step begins —
+	 * to drain messages to inject alongside the tool results. Whatever it
+	 * returns is appended as user-role messages to the next step's input, so
+	 * a caller can inject mid-turn guidance the model sees with the tool
+	 * results. When omitted or returning an empty array, no injection happens
+	 * (the runtime is unchanged).
+	 *
+	 * Injected (not ambient) so the kernel stays pure: it owns no queue and
+	 * names no feature — it just calls the callback and appends what it gets.
+	 * Only invoked when a step PRODUCED tool calls (the tool-result boundary);
+	 * a step that ends without tool calls does not drain (the caller decides
+	 * what to do with any pending messages after the turn ends).
+	 */
+	readonly drainSteering?: () => readonly ChatMessage[];
 }
 
 /**