feat(web): list background tasks in the session header

The task registry has run every background bash, pwsh, pty-send, and
one-shot subagent since it landed, but only the model could read it: a
human at the Web client could not see that a build was running, tell a
finished task from a stuck one, or find its outcome anywhere but the
`run_in_background` tool card that printed an id and never updated.

Task state now reaches the browser as one whole-snapshot `session/tasks`
mux frame per session, pushed at every registry commit that changes what
that session can see. `TaskService` gains `onTasksChanged`, which is
owner-granular because owner-disposal removal is a change no per-task
record can express. The carrier reads the exact owner the listener hands
it, so a push stays correct while that scope tears down, and reads the
baseline through the non-resuming `ctx.agents.get` so listing never
revives a cold session. The client keeps a last-wins mirror on
`SessionListState`, and a new `dsh-client-ui-task` package renders it
beside the subagent catalog — rendering nothing at all until the session
has a task, so an ordinary conversation grows no new chrome.

Streamed per-task output and human-initiated cancellation are separate
phases; the note records why neither has to undo this channel, and why
no Web path may call the consuming `ctx.tasks.read()`.

packages/tasks/tasks/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/tasks/tasks/README.md
-README.md: 2f822bad139020f0ebae0165aa4e8893853f635d
-README.zh.md: fdc619fbb46267b2ae550c85cb14fcf8a916f638
+README.md: e7431cca0956429788274b0d44a249dd1e374156
+README.zh.md: e8c1162f66471eee3c04111c8617dd84295dc6cf

packages/tasks/tasks/README.md

@@ -12,6 +12,7 @@ The background task registry seam (`ctx.tasks`). The abstract `TaskService` and
 - `kill(id, caller?, reason?)` invokes producer cancellation before changing status. A cancellation throw leaves the task running; success changes it to `stopping` and marks terminal delivery reported.
 - `wait(id, timeoutMs, caller?, signal?)` returns a terminal snapshot or the live snapshot at timeout. Aborting stops only the wait; settlement wins once it has committed terminal delivery to that waiter.
 - `onTaskDone(listener)` observes each terminal record with the exact owner. Listener throws and rejections are contained; listener work is not awaited.
+- `onTasksChanged(listener)` observes visible-set changes — registration, the stopping transition, settlement, and owner-disposal removal — carrying only the owner whose set moved, or `undefined` when an unowned task changed and every caller's set moved with it. It is owner-granular because removal is a change no per-task record can express, and it is not a superset of `onTaskDone`: it carries no delivery meaning and marks nothing reported.
 - `attachSurface(name)` declares a control surface for its effect lifetime. `start()` fails before producer execution when none is attached.
 
 Owned access compares the task's `SessionId` with the caller's. Ids such as `bash-1` are predictable, so this fence is the boundary. Unowned tasks are open to callers and last until service disposal.

packages/tasks/tasks/README.zh.md

@@ -12,6 +12,7 @@
 - `kill(id, caller?, reason?)` 在更改状态前调用生产方取消。取消抛出异常时任务保持运行；成功则把状态改为 `stopping`，并将终止交付标记为已报告。
 - `wait(id, timeoutMs, caller?, signal?)` 返回终止快照，或在超时时返回存活快照。中止只会停止等待；一旦终止交付已向该等待方提交，终止结果优先。
 - `onTaskDone(listener)` 观察每条终止记录及其精确 owner。监听器抛出的异常和产生的拒绝都会被隔离；系统不会等待监听器工作。
+- `onTasksChanged(listener)` 观察可见集合的变化——注册、转入 stopping、结算，以及 owner 销毁时的移除——只携带集合发生变化的那个 owner，或在无主任务变化、因而每个调用方的集合都随之变化时携带 `undefined`。它按 owner 分粒度，因为移除是任何逐任务记录都无法表达的变化；它也不是 `onTaskDone` 的超集：它不含任何投递含义，也不把任何东西标为已上报。
 - `attachSurface(name)` 在其 effect 生命周期内声明控制表层。如果没有附加任何表层，`start()` 会在生产方执行前失败。
 
 有 owner 的访问会比较任务的 `SessionId` 与调用方。`bash-1` 等 id 可预测，因此这道隔离是安全边界。无 owner 的任务向调用方开放，并持续到服务释放。

packages/tasks/tasks/package.json

@@ -15,12 +15,17 @@
       "types": "./lib/types/invariant.d.ts",
       "default": "./lib/invariant.js"
     },
+    "./brand": {
+      "types": "./lib/types/brand.d.ts",
+      "default": "./lib/types/brand.js"
+    },
     "./src/*": "./src/*",
     "./package.json": "./package.json"
   },
   "files": [
     "lib/index.js",
     "lib/invariant.js",
+    "lib/types/**/*.js",
     "lib/types/**/*.d.ts"
   ],
   "license": "BSD-3-Clause",

packages/tasks/tasks/src/brand.ts

@@ -0,0 +1,28 @@
+/**
+ * dsh-tasks' owned branded id, carried across the registry, the model-facing
+ * control surface, and the client wire.
+ *
+ * It lives in its own leaf because the package root and `./types` both reach
+ * `dsh-agent` through the owner and listener signatures, which a Client program
+ * cannot resolve even as a type. A browser-safe consumer imports the id here;
+ * `Branded<B>` itself comes from the zero-dependency `@deepseek-ai/dsh-brand`.
+ *
+ * @module @deepseek-ai/dsh-tasks/brand
+ */
+
+import type { Branded } from '@deepseek-ai/dsh-brand'
+
+/**
+ * Identifies a background task. The registry generates `<kind>-N`; predictable
+ * ids rely on owner authorization rather than secrecy.
+ */
+export type TaskId = Branded<'TaskId'>
+
+/**
+ * Brand a string as a {@link TaskId}.
+ * @param id - the raw task-id string (the registry generates `<kind>-N`).
+ * @returns the same string, branded; no validation is performed.
+ */
+export function TaskId(id: string): TaskId {
+  return id as TaskId
+}

packages/tasks/tasks/src/index.ts

@@ -8,7 +8,9 @@
 
 import { Context, Service } from 'cordis'
 import type { Agent } from '@deepseek-ai/dsh-agent'
-import type { TaskDoneListener, TaskId, TaskRead, TaskSnapshot, TaskStart } from './types.ts'
+import type {
+  TaskDoneListener, TaskId, TaskRead, TaskSnapshot, TaskStart, TasksChangedListener,
+} from './types.ts'
 
 export { TaskId } from './types.ts'
 export type {
@@ -21,6 +23,7 @@ export type {
   TaskSnapshot,
   TaskStart,
   TaskStatus,
+  TasksChangedListener,
 } from './types.ts'
 
 declare module 'cordis' {
@@ -129,6 +132,22 @@ export abstract class TaskService extends Service {
    */
   abstract onTaskDone(listener: TaskDoneListener): () => void
 
+  /**
+   * Register an effect-scoped observer of visible-set changes. It fires after
+   * every commit that changes what {@link list} returns for that owner —
+   * registration, the stopping transition, settlement, and owner-disposal
+   * removal — so an observer re-reads rather than accumulating deltas.
+   *
+   * This is not a superset of {@link onTaskDone}: that one delivers the terminal
+   * record under first-wins semantics a control surface couples to notice
+   * delivery, while this one carries no delivery meaning and marks nothing
+   * reported. Listeners are contained and never awaited.
+   * @param listener - receives the owner whose visible set changed, or
+   *   `undefined` when an unowned task changed and every caller's set did.
+   * @returns disposer that unregisters the listener.
+   */
+  abstract onTasksChanged(listener: TasksChangedListener): () => void
+
   /**
    * Attach an effect-scoped surface that can read and stop tasks. {@link start}
    * refuses work while none is attached.

packages/tasks/tasks/src/types.ts

@@ -4,24 +4,11 @@
  * @module @deepseek-ai/dsh-tasks/types
  */
 
-import type { Branded } from '@deepseek-ai/dsh-brand'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import type { SessionId } from '@deepseek-ai/dsh-session'
+import type { TaskId } from './brand.ts'
 
-/**
- * Identifies a background task. The registry generates `<kind>-N`; predictable
- * ids rely on owner authorization rather than secrecy.
- */
-export type TaskId = Branded<'TaskId'>
-
-/**
- * Brand a string as a {@link TaskId}.
- * @param id - the raw task-id string (the registry generates `<kind>-N`).
- * @returns the same string, branded; no validation is performed.
- */
-export function TaskId(id: string): TaskId {
-  return id as TaskId
-}
+export { TaskId } from './brand.ts'
 
 /**
  * Task lifecycle: `running`, optionally `stopping`, then exactly one terminal
@@ -157,3 +144,14 @@ export type TaskDoneListener = (
   snapshot: TaskSnapshot,
   owner: Agent | undefined,
 ) => void | PromiseLike<void>
+
+/**
+ * Observation callback for a change to what one owner's {@link TaskService.list}
+ * would return. It is owner-granular rather than task-granular because the
+ * change may be a removal, which no per-task record can express, and because
+ * its consumers re-read the whole visible set anyway.
+ *
+ * An `undefined` owner means an unowned task changed, so every caller's visible
+ * set changed with it.
+ */
+export type TasksChangedListener = (owner: Agent | undefined) => void

packages/tasks/tasks/tests/service.spec.ts

@@ -2,7 +2,9 @@ import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import { TaskId, TaskService } from '@deepseek-ai/dsh-tasks'
-import type { TaskDoneListener, TaskRead, TaskSnapshot, TaskStart } from '@deepseek-ai/dsh-tasks'
+import type {
+  TaskDoneListener, TaskRead, TaskSnapshot, TaskStart, TasksChangedListener,
+} from '@deepseek-ai/dsh-tasks'
 
 /**
  * Minimal concrete registry: one canned record. The seam owns the contract
@@ -50,6 +52,10 @@ class StubTaskService extends TaskService {
     return () => {}
   }
 
+  onTasksChanged(_listener: TasksChangedListener): () => void {
+    return () => {}
+  }
+
   attachSurface(_name: string): () => void {
     return () => {}
   }
@@ -70,6 +76,8 @@ describe('TaskService seam', () => {
     await expect(ctx.tasks.wait(id, 5)).resolves.toMatchObject({ id })
     const detachListener = ctx.tasks.onTaskDone(() => {})
     detachListener()
+    const detachChanges = ctx.tasks.onTasksChanged(() => {})
+    detachChanges()
     detachSurface()
   })