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()`.

docs/core-data-structures/tasks.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 docs/core-data-structures/tasks.md
-tasks.md: a38055d3ef7aa18e62678f92eb5ac5ae2a09c205
-tasks.zh.md: f34d42e713c3a0c11cbf88d52e573bb100c52493
+tasks.md: e07fa4798d9d7585fd1036346f8df39068884835
+tasks.zh.md: d958be308f462db371d10240cfa369a892fe1ce0

docs/core-data-structures/tasks.md

@@ -151,4 +151,4 @@ interface TaskRead {
 
 ## Service behavior
 
-The abstract [`TaskService`](../../packages/tasks/tasks/src/index.ts) seam defines atomic `start`, caller-scoped `get` and `list`, `read`, `kill`, bounded `wait`, contained `onTaskDone` listeners, and the `attachSurface` availability fence; [`LocalTaskService`](../../packages/tasks/tasks-local/src/index.ts) is the process-local implementation. Authorization compares owner sessions; owner cleanup selects the exact registered `Agent` instance. See [`dsh-tasks`](../../packages/tasks/tasks/README.md) for the seam contract, [`dsh-tasks-local`](../../packages/tasks/tasks-local/README.md) for the registry lifecycle, and [`dsh-tool-tasks`](../../packages/tasks/tool-tasks/README.md) for the model-facing surface.
+The abstract [`TaskService`](../../packages/tasks/tasks/src/index.ts) seam defines atomic `start`, caller-scoped `get` and `list`, `read`, `kill`, bounded `wait`, contained `onTaskDone` and `onTasksChanged` listeners, and the `attachSurface` availability fence; [`LocalTaskService`](../../packages/tasks/tasks-local/src/index.ts) is the process-local implementation. The two listener kinds are not nested: `onTaskDone` delivers one terminal record under first-wins semantics a control surface couples to notice delivery, while `onTasksChanged` observes every visible-set change — 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. Authorization compares owner sessions; owner cleanup selects the exact registered `Agent` instance. See [`dsh-tasks`](../../packages/tasks/tasks/README.md) for the seam contract, [`dsh-tasks-local`](../../packages/tasks/tasks-local/README.md) for the registry lifecycle, and [`dsh-tool-tasks`](../../packages/tasks/tool-tasks/README.md) for the model-facing surface.

docs/core-data-structures/tasks.zh.md

@@ -151,4 +151,4 @@ interface TaskRead {
 
 ## 服务行为
 
-抽象的 [`TaskService`](../../packages/tasks/tasks/src/index.ts) seam 定义原子 `start`、限定调用方作用域的 `get` 和 `list`、`read`、`kill`、有界 `wait`、故障隔离的 `onTaskDone` 监听器，以及 `attachSurface` 可用性防线；[`LocalTaskService`](../../packages/tasks/tasks-local/src/index.ts) 是其进程局部实现。授权会比较拥有者会话；拥有者清理会选择确切的已注册 `Agent` 实例。seam 契约见 [`dsh-tasks`](../../packages/tasks/tasks/README.md)，注册表生命周期见 [`dsh-tasks-local`](../../packages/tasks/tasks-local/README.md)，面向模型的接口见 [`dsh-tool-tasks`](../../packages/tasks/tool-tasks/README.md)。
+抽象的 [`TaskService`](../../packages/tasks/tasks/src/index.ts) seam 定义原子 `start`、限定调用方作用域的 `get` 和 `list`、`read`、`kill`、有界 `wait`、故障隔离的 `onTaskDone` 与 `onTasksChanged` 监听器，以及 `attachSurface` 可用性防线；[`LocalTaskService`](../../packages/tasks/tasks-local/src/index.ts) 是其进程局部实现。两类监听器不是包含关系：`onTaskDone` 按控制面与通知投递绑定的 first-wins 语义投递唯一一条终态记录，而 `onTasksChanged` 观察每一次可见集合的变化——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`。授权会比较拥有者会话；拥有者清理会选择确切的已注册 `Agent` 实例。seam 契约见 [`dsh-tasks`](../../packages/tasks/tasks/README.md)，注册表生命周期见 [`dsh-tasks-local`](../../packages/tasks/tasks-local/README.md)，面向模型的接口见 [`dsh-tool-tasks`](../../packages/tasks/tool-tasks/README.md)。