feat(message-queue): cancel a queued steering message by id

Add the ability to cancel/close a single queued message so it never runs:
while a turn is GENERATING and a user message sits in the steering queue
(waiting for delivery at the next tool-result boundary or carry into a new
turn), a client can remove one message by id. The cancelled message is never
delivered as steering and never carried into a new turn. Complements the
existing chat.queue enqueue (enqueue adds; cancel removes one).

Layers (all additive; nothing existing breaks):

- message-queue pure core: `cancel(state, conversationId, messageId)` —
  splices a single message out by id, returns the post-cancel snapshot.
  Idempotent (no-op if not found). Drops the key when the queue empties
  (mirrors drain).
- message-queue service: `MessageQueueService.cancel()` — wraps the pure op,
  pushes a surface update ONLY on a real change (queue shrank); a missing-id
  cancel is a no-op with no surface push (mirrors drain's no-notify-on-empty).
- session-orchestrator: `cancelQueuedMessage({ conversationId, messageId })`
  -> `{ cancelled, queue }`. The single entry transports call; resolves the
  queue lazily (same as enqueue). Degrades to `{ cancelled: false, queue: [] }`
  when the message-queue extension isn't loaded. `cancelled` is derived from
  the queue length delta (true iff a message was removed).

API contract (documented for the frontend agent; see
frontend-cancel-queued-message-handoff.md):

  HTTP:  DELETE /conversations/:id/queue/:messageId
         -> 200 QueueCancelResponse { conversationId, cancelled, queue }
         (cancelled:false is a 200 idempotent no-op, not an error)

  WS:    chat.queue.cancel { type:"chat.queue.cancel", conversationId, messageId }
         (additive to WsClientMessage). Fire-and-forget like chat.queue: success
         is confirmed by the message-queue SURFACE updating (the cancelled
         message leaves payload.messages). A missing-id cancel is a silent
         no-op (no surface update, no error). Malformed (empty conversationId/
         messageId) -> chat.error.

  No new AgentEvent: a cancelled message never appears in the transcript (it
  never runs). The existing message-queue surface already reflects the
  post-cancel snapshot. Race-safe by construction: if the kernel drains the
  queue (steering) or carries it into a new turn before the cancel runs, the
  message is already gone -> cancel returns cancelled:false (a no-op).

Version bump: @dispatch/transport-contract 0.23.0 -> 0.24.0 (additive:
QueueCancelResponse + ChatQueueCancelMessage added to WsClientMessage).
@dispatch/wire unchanged (QueuedMessage.id is the cancel target).

No CLI command added: the CLI has no queue-listing affordance to discover a
messageId, so a CLI cancel would have no input source. The HTTP DELETE is
available for any non-WS client that knows the id (e.g. from a prior enqueue
response's queue[]).

Verification: tsc -b EXIT 0; vitest 2024 passed / 6 skipped (25 new tests);
biome EXIT 0 (0 errors).

1 files changed, 69 insertions, 0 deletions

diff --git a/packages/message-queue/src/service.test.ts b/packages/message-queue/src/service.test.ts
index aa59dd3..086414e 100644
--- a/packages/message-queue/src/service.test.ts
+++ b/packages/message-queue/src/service.test.ts
@@ -99,3 +99,72 @@ describe("message-queue service", () => {
     expect(combine(drained)).toBe("alpha\n\nbeta");
   });
 });
+
+describe("message-queue service cancel", () => {
+  it("cancel removes the message and pushes a surface update (queue shrank)", () => {
+    const deps = makeDeps();
+    const svc = createMessageQueueService(deps);
+    svc.enqueue("c1", "a"); // q-1
+    svc.enqueue("c1", "b"); // q-2
+    svc.enqueue("c1", "c"); // q-3
+    expect(deps.calls.value).toBe(3); // three enqueues notified
+
+    const snapshot = svc.cancel("c1", "q-2");
+    expect(deps.calls.value).toBe(4); // cancel pushed a surface update
+    expect(snapshot.map((m) => m.id)).toEqual(["q-1", "q-3"]);
+
+    // live state reflects the removal
+    expect(svc.getQueue("c1").map((m) => m.id)).toEqual(["q-1", "q-3"]);
+  });
+
+  it("cancel of the only message empties the queue + pushes a surface update", () => {
+    const deps = makeDeps();
+    const svc = createMessageQueueService(deps);
+    svc.enqueue("c1", "only"); // q-1
+    expect(deps.calls.value).toBe(1);
+
+    const snapshot = svc.cancel("c1", "q-1");
+    expect(deps.calls.value).toBe(2); // surface update (queue → empty)
+    expect(snapshot).toEqual([]);
+    expect(svc.getQueue("c1")).toEqual([]);
+  });
+
+  it("cancel of a missing id does NOT push a surface update (no change)", () => {
+    const deps = makeDeps();
+    const svc = createMessageQueueService(deps);
+    svc.enqueue("c1", "a"); // q-1
+    expect(deps.calls.value).toBe(1);
+
+    // unknown message id — no-op, no notify
+    const snapshot = svc.cancel("c1", "nope");
+    expect(deps.calls.value).toBe(1); // unchanged — no change
+    expect(snapshot.map((m) => m.id)).toEqual(["q-1"]);
+
+    // unknown conversation — also a no-op, no notify
+    expect(svc.cancel("nope", "q-1")).toEqual([]);
+    expect(deps.calls.value).toBe(1); // still unchanged
+  });
+
+  it("cancel after a drain (queue empty) is a no-op with no surface update", () => {
+    const deps = makeDeps();
+    const svc = createMessageQueueService(deps);
+    svc.enqueue("c1", "a"); // q-1
+    svc.drain("c1");
+    expect(deps.calls.value).toBe(2); // enqueue + drain
+
+    expect(svc.cancel("c1", "q-1")).toEqual([]);
+    expect(deps.calls.value).toBe(2); // no notify — queue was already empty
+  });
+
+  it("cancel is scoped per conversation", () => {
+    const deps = makeDeps();
+    const svc = createMessageQueueService(deps);
+    svc.enqueue("c1", "a"); // q-1
+    svc.enqueue("c2", "b"); // q-2
+
+    const snapshot = svc.cancel("c1", "q-1");
+    expect(snapshot).toEqual([]);
+    expect(svc.getQueue("c1")).toEqual([]);
+    expect(svc.getQueue("c2").map((m) => m.id)).toEqual(["q-2"]);
+  });
+});