fix(tasks-local): layer control surfaces and listeners by registering scope

One host registry serves every composition in the process, so its two
service-wide collections answered per-owner questions process-wide. `start()`
asked only whether SOME surface was attached, so an agent whose own composition
loads no `tool-tasks` could start work it has no tool to collect or stop as soon
as any other preset attached one — and the answer changed depending on which
sessions happened to be open. `settle()` walked every registered listener, so a
task settling without a waiter injected one completion notice per mounted
preset into the same owner.

Both collections now sit in `ScopedLayers`, the layered-registry primitive
`tools` and `skills` already use: a registration files into its registering
context's scope, and a read unions the global layer with the owner's scope
chain. A surface or listener registered from an unscoped context lands in the
global layer and serves every owner, which is exactly the host-plane
composition's own controls, so the TUI path is unchanged without a special
case.

This supersedes the consumer-side filter in the previous commit. That filter
produced the right notices but sat in the wrong layer: it left the `start()`
gate process-wide, it could not be enforced against a producer that resolves
the registry directly, and it made a Consumer carry scope knowledge that the
other layered registries keep in the registry. `tool-tasks` is scope-agnostic
again and the `dsh-scope` edge moves to `tasks-local`.

`start()`'s refusal is now owner-relative, so its model-visible text names the
agent rather than the process. The shipped `minimal` preset keeps
`enableRunInBackground: false`, no longer as the safety boundary — the registry
owns that now — but so an agent that could never collect a task is not offered
the parameter at all.

Refs #2141

.agents/notes/implemented/architecture/2026-07-26-task-registry-seam.md

@@ -12,7 +12,7 @@ The [background-task runtime](2026-06-20-generic-long-running-tool-runtime.md) s
 
 `tasks/` is now a three-package capability family in the bash-trio shape:
 
-- **`@deepseek-ai/dsh-tasks` (Service Definition)** — the abstract `TaskService extends Service` owning `ctx.tasks`, the eight-method contract (`start`, `list`, `get`, `read`, `kill`, `wait`, `onTaskDone`, `attachSurface`), all vocabulary types (`TaskId`, `TaskKindMap`, `TaskStart`, `TaskHooks`, `TaskOutcome`, `TaskSnapshot`, `TaskRead`, `TaskDoneListener`), and the snapshot invariant companion. The class-level JSDoc states the semantics every Service provider owes: registrations outlive producer and surface fibers, owned access is session-fenced, settlement is first-wins with contained listeners, and `start` refuses work while no control surface is attached.
+- **`@deepseek-ai/dsh-tasks` (Service Definition)** — the abstract `TaskService extends Service` owning `ctx.tasks`, the eight-method contract (`start`, `list`, `get`, `read`, `kill`, `wait`, `onTaskDone`, `attachSurface`), all vocabulary types (`TaskId`, `TaskKindMap`, `TaskStart`, `TaskHooks`, `TaskOutcome`, `TaskSnapshot`, `TaskRead`, `TaskDoneListener`), and the snapshot invariant companion. The class-level JSDoc states the semantics every Service provider owes: registrations outlive producer and surface fibers, owned access is session-fenced, settlement is first-wins with contained listeners, and `start` refuses work while no attached control surface serves the spec's owner (surfaces and listeners are scope-layered, so one process-wide registry answers both questions per owner).
 - **`@deepseek-ai/dsh-tasks-local` (Service provider)** — `LocalTaskService`, the process-local registry moved verbatim: the in-memory store, per-kind counters, waiter bookkeeping, `TASK_WAIT_TIMEOUT` deadline code, owner-cleanup effects, and force-fail teardown. The `dsh-timeout` dependency moves here with it; the Service Definition package has no provider dependencies.
 - **`@deepseek-ai/dsh-tool-tasks` (Consumer)** — unchanged; it injects `'tasks'` and never imports provider types.