fix(tools): restrict what a scope inherits, not just the global layer

A restriction was compiled against the global tool layer alone: only
global-layer tools were tested against `admits()`, and every chain-layer
tool was overlaid unfiltered afterward. That read the exempt set as "the
global layer" when what it means is "what this scope registers itself" —
two descriptions of the same set only while every model-facing tool sat in
the host composition.

Moving those rows onto the agent plane separated them. A preset's tools are
an ANCESTOR contribution to a joined agent, so a subagent's `toolFilter`
stopped constraining anything it was given; and with the global layer empty
`restrict()` rejected every name it received as unknown, failing the child
outright. With the same tools in the global layer the filter still admits
and applies normally, which is what makes this a regression of the move
rather than a standing limitation.

`view()` now filters everything a scope inherits — the global layer and
every ancestor layer on its chain — and exempts only the layer the scope
owns. That exemption is load-bearing rather than incidental: the delegation
runtime registers a child's `report` and structured-output tools into the
child's own layer, and a filter naming the capabilities the child may use
must not strip the machinery it answers through. Tool order, and with it
prefix-cache reuse, is unchanged: inherited names keep their global-then-
ancestor position and own-layer names still come last.

The diagnostic said "unknown global tool" while listing what is really the
inherited surface; it now names the surface it checks and says why an
own-layer name is not restrictable.

Fixes #2185

docs/subsystems/tools.md

@@ -150,19 +150,20 @@ type InferArgs<S> = InferProperties<S, []>
 
 Registration is a trusted same-process contract. The registry borrows the typed definition as readonly input, requires `output`, validates its raw schema, and checks semantic requirements such as a positive finite `timeoutMs`; `schemas()` constructs the model-facing projection when building a request, so execution and presentation share one resolved definition without leaking callbacks onto the wire.
 
-## `ToolRestriction` — one scope's live global filter
+## `ToolRestriction` — one scope's live filter over what it inherits
 
-`ToolRestriction` applies only to the live deployment-global tool layer. The registry compiles readonly names into private sets, intersects multiple restrictions, then overlays scope-local tools. A deny-only filter admits later unlisted globals, while an allow-list excludes them.
+`ToolRestriction` applies to the tools a scope inherits: the deployment-global layer plus every ancestor scope on its chain. The registry compiles readonly names into private sets, intersects multiple restrictions, then overlays the scope's OWN registrations, which stay exempt so a delegated child keeps the tools it answers through. A deny-only filter admits later unlisted inherited tools, while an allow-list excludes them.
 
 ```ts type-equiv
 /**
- * Per-scope filter over global tools. Restrictions intersect and do not affect
- * scoped registrations or the reserved Code Mode transport.
+ * Per-scope filter over the tools a scope INHERITS — the global layer and
+ * every ancestor layer on its chain. Restrictions intersect, and do not affect
+ * the scope's own registrations or the reserved Code Mode transport.
  */
 interface ToolRestriction {
-  /** Global tool names that stay visible; everything else is removed. */
+  /** Inherited tool names that stay visible; every other inherited one is removed. */
   readonly allow?: readonly string[]
-  /** Global tool names removed from visibility. */
+  /** Inherited tool names removed from visibility. */
   readonly deny?: readonly string[]
 }
 ```
@@ -565,7 +566,7 @@ async execute(exec: ToolExecutionInput): Promise<ToolExecutionResult>
 
 Types: [ScopeKey](scope.md)
 
-Source: [`packages/core/tools/src/index.ts:760`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:761`](../../packages/core/tools/src/index.ts)
 
 <a id="tools-events"></a>