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, 13 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index bf57854..228ef8a 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -538,6 +538,19 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 
 			if (step === MAX_STEPS - 1) {
 				finishReason = "max-steps";
+				// No next step → no tool-result boundary. Leave any pending
+				// steering messages for the caller (it owns the queue).
+			} else {
+				// Tool-result boundary: this step produced tool calls and we are
+				// about to call provider.stream again. Drain steering messages
+				// and append them after the tool results, before the next call.
+				// The kernel owns no queue and names no feature — it just calls
+				// the callback and appends. Emits nothing (caller emits the
+				// `steering` AgentEvent in its own wrapper).
+				const steering = input.drainSteering?.() ?? [];
+				for (const msg of steering) {
+					messages.push(msg);
+				}
 			}
 		}
 	} finally {