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

packages/tasks/tool-tasks/README.md

@@ -20,7 +20,7 @@ When a producer supplies `outputLimitBytes`, `task_output`, terminal `task_kill`
 
 An unreported completion injects `background task <id> (<kind>: <label>) finished [status: ...]. Read its output with task_output.` into the exact owner's next-step inbox. When bounded, the stable id prefix and collection command outrank variable label/detail so the notice remains actionable at PTY's supported 64-byte minimum. Injection is durable pending context for a later pre-step claim, not a wake-up; cancellation or owner disposal may discard it before claim. A kill or terminal read/wait marks delivery reported and suppresses the redundant notice.
 
-One host registry may carry several mounts of this plugin — one per agent preset — and the registry broadcasts each settlement to every mount. A scoped mount delivers only to owners composed under its own scope, so an agent reads exactly one notice per completion however many presets are mounted; an unscoped mount is the host-plane instance and delivers to every owner.
+One host registry may carry several mounts of this plugin — one per agent preset. The registry routes each settlement to the listeners the owner's scope chain reaches, so a mount under one preset never sees another preset's agents and an agent reads exactly one notice per completion however many presets are mounted. The same routing decides which agents this mount's control surface serves: an agent whose composition loads no `tool-tasks` cannot start background work at all.
 
 ## Config