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, 82 insertions, 0 deletions

diff --git a/packages/message-queue/src/extension.ts b/packages/message-queue/src/extension.ts
new file mode 100644
index 0000000..26d19ca
--- /dev/null
+++ b/packages/message-queue/src/extension.ts
@@ -0,0 +1,82 @@
+/**
+ * message-queue extension — owns the per-conversation steering message queue
+ * (state + surface + drain-and-clear). Plugs into the kernel host via a
+ * manifest + activate(host); the session-orchestrator drains it via the
+ * `messageQueueHandle` service. Does NOT touch the turn loop.
+ */
+import type { Extension, HostAPI, Manifest } from "@dispatch/kernel";
+import type { SurfaceContext, SurfaceProvider } from "@dispatch/surface-registry";
+import { surfaceRegistryHandle } from "@dispatch/surface-registry";
+import type { SurfaceSpec } from "@dispatch/ui-contract";
+import { buildQueueSpec, MESSAGE_QUEUE_SURFACE_ID } from "./pure.js";
+import { createMessageQueueService, messageQueueHandle } from "./service.js";
+
+export const manifest: Manifest = {
+	id: "message-queue",
+	name: "Message Queue",
+	version: "0.0.0",
+	apiVersion: "^0.1.0",
+	trust: "bundled",
+	activation: "eager",
+	dependsOn: ["surface-registry"],
+	capabilities: {},
+	contributes: {
+		services: ["message-queue"],
+	},
+};
+
+export function activate(host: HostAPI): void {
+	const registry = host.getService(surfaceRegistryHandle);
+
+	const subscribers = new Set<() => void>();
+
+	const service = createMessageQueueService({
+		id: () => crypto.randomUUID(),
+		now: () => Date.now(),
+		notify: () => {
+			for (const sub of subscribers) {
+				sub();
+			}
+		},
+		logger: host.logger,
+	});
+
+	host.provideService(messageQueueHandle, service);
+
+	function getSpec(context?: SurfaceContext): SurfaceSpec {
+		const convId = context?.conversationId;
+		const messages = convId === undefined ? [] : service.getQueue(convId);
+		return buildQueueSpec(messages);
+	}
+
+	function invoke(_actionId: string, _payload?: unknown, _context?: SurfaceContext): void {
+		// The message-queue surface is read-only: a client renders the queue and
+		// the session-orchestrator drains it. No client-facing actions.
+	}
+
+	const provider: SurfaceProvider = {
+		catalogEntry: {
+			id: MESSAGE_QUEUE_SURFACE_ID,
+			region: "side",
+			title: "Message Queue",
+			scope: "conversation",
+		},
+		getSpec,
+		invoke,
+		subscribe(onChange) {
+			subscribers.add(onChange);
+			return () => {
+				subscribers.delete(onChange);
+			};
+		},
+	};
+
+	registry.register(provider);
+
+	host.logger.info("message-queue: registered");
+}
+
+export const extension: Extension = {
+	manifest,
+	activate,
+};