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, 79 insertions, 0 deletions

diff --git a/packages/message-queue/src/pure.test.ts b/packages/message-queue/src/pure.test.ts
index 3fd6039..4fca0fa 100644
--- a/packages/message-queue/src/pure.test.ts
+++ b/packages/message-queue/src/pure.test.ts
@@ -2,6 +2,7 @@ import type { QueuedMessage } from "@dispatch/wire";
 import { describe, expect, it } from "vitest";
 import {
   buildQueueSpec,
+  cancel,
   combine,
   drain,
   enqueue,
@@ -98,6 +99,84 @@ describe("drain", () => {
   });
 });
 
+describe("cancel", () => {
+  it("removes the matching message and returns the post-cancel snapshot", () => {
+    const state: MessageQueueState = new Map();
+    const deps = makeDeps();
+    enqueue(state, "c1", "a", deps); // id-1
+    enqueue(state, "c1", "b", deps); // id-2
+    enqueue(state, "c1", "c", deps); // id-3
+
+    const snapshot = cancel(state, "c1", "id-2");
+    expect(snapshot.map((m) => m.id)).toEqual(["id-1", "id-3"]);
+    expect(snapshot.map((m) => m.text)).toEqual(["a", "c"]);
+    // live state reflects the removal
+    expect(getQueue(state, "c1").map((m) => m.id)).toEqual(["id-1", "id-3"]);
+  });
+
+  it("removing the only message drops the key (queue is empty + clean)", () => {
+    const state: MessageQueueState = new Map();
+    const deps = makeDeps();
+    enqueue(state, "c1", "only", deps); // id-1
+
+    const snapshot = cancel(state, "c1", "id-1");
+    expect(snapshot).toEqual([]);
+    expect(getQueue(state, "c1")).toEqual([]);
+    // key removed so a fresh getQueue is a clean empty (not a lingering [] key)
+    expect(state.has("c1")).toBe(false);
+  });
+
+  it("returns a COPY — mutating the snapshot does not affect live state", () => {
+    const state: MessageQueueState = new Map();
+    const deps = makeDeps();
+    enqueue(state, "c1", "a", deps);
+    enqueue(state, "c1", "b", deps);
+
+    const snapshot = cancel(state, "c1", "id-1");
+    snapshot.push({ id: "evil", text: "mutate", queuedAt: 0 });
+    expect(getQueue(state, "c1")).toHaveLength(1);
+  });
+
+  it("is idempotent — cancelling a missing id is a no-op (returns snapshot without it)", () => {
+    const state: MessageQueueState = new Map();
+    const deps = makeDeps();
+    enqueue(state, "c1", "a", deps); // id-1
+
+    // unknown message id
+    const snapshot = cancel(state, "c1", "nope");
+    expect(snapshot.map((m) => m.id)).toEqual(["id-1"]);
+    expect(getQueue(state, "c1")).toHaveLength(1);
+
+    // a second cancel of the already-removed id-1 (re-add then cancel twice)
+    cancel(state, "c1", "id-1");
+    expect(getQueue(state, "c1")).toEqual([]);
+    expect(cancel(state, "c1", "id-1")).toEqual([]); // already gone — no-op
+  });
+
+  it("is scoped per conversation — cancelling on one conversation does not affect another", () => {
+    const state: MessageQueueState = new Map();
+    const deps = makeDeps();
+    enqueue(state, "c1", "a", deps); // id-1
+    enqueue(state, "c2", "b", deps); // id-2
+
+    const snapshot = cancel(state, "c1", "id-1");
+    expect(snapshot).toEqual([]);
+    expect(getQueue(state, "c1")).toEqual([]);
+    // c2 untouched
+    expect(getQueue(state, "c2").map((m) => m.id)).toEqual(["id-2"]);
+  });
+
+  it("cancelling on an unknown / empty conversation is a no-op (returns [])", () => {
+    const state: MessageQueueState = new Map();
+    expect(cancel(state, "unknown", "anything")).toEqual([]);
+    // unknown id on a conversation that exists but is empty post-drain
+    const deps = makeDeps();
+    enqueue(state, "c1", "a", deps);
+    drain(state, "c1"); // empties + deletes the key
+    expect(cancel(state, "c1", "id-1")).toEqual([]);
+  });
+});
+
 describe("combine", () => {
   it("combine joins texts with blank-line separator", () => {
     const msgs: QueuedMessage[] = [