refactor(scope,agent-presets): per-preset standing mounts over a scope parent chain

A preset is now ONE composition per process, not one per session. The roster
mounts it once under a synthetic standing scope; each agent joins by having
its scope key parented to the mount's. Two mechanisms in dsh-scope carry the
whole change: registration views walk the parent chain (global → preset →
agent, nearest shadowing farthest — ScopedLayers.chainLayers), and scoped
event dispatch admits a listener tagged with an ancestor of the carrier key,
which is what lets a standing composition's plan/compaction/token listeners
observe each agent composed under it while a sibling preset's stay deaf.

The preset plugins already key their state by Session/Agent — they predate
presets and were written for the shared world — so sharing one instance is a
return to their design, not a rewrite. Preset ymls are unchanged: one mount
per preset means one Entry per preset, whose entry-local realms keep two
presets' services apart exactly as they kept two sessions' apart before.

The standing scope hangs off the service's UNTRACED context (selfCtx): a
method invoked through the traceable proxy sees this.ctx rebound to the
caller and carrying its shadow, and a subtree minted from that resolves every
service through the shadow's fiber instead of each entry's own inject store —
preset rows then fail on the very services they declare.

A standing mount survives its agents deliberately. The composition a running
session joined must outlive the file changing or disappearing underneath it;
reclamation happens at whole-tree teardown, and file edits reach only future
generations (the authoring layer swaps the pointer, never disposes a joined
generation).

packages/core/system-prompt/src/index.ts

@@ -429,9 +429,11 @@ export class SystemPrompt extends Service {
     for (const [name, provider] of this.layers.global.variables.entries()) {
       variables[name] = provider(context)
     }
-    const scopedVariables = this.layers.peek(scope)?.variables
-    for (const [name, provider] of scopedVariables?.entries() ?? []) {
-      variables[name] = provider(context)
+    // Scope-chain variables, farthest first, so the nearest scope wins a name.
+    for (const layer of this.layers.chainLayers(scope)) {
+      for (const [name, provider] of layer.variables.entries()) {
+        variables[name] = provider(context)
+      }
     }
     // Scoped sections shadow globals before the stable order sort.
     const sectionByName = this.layers.merge(scope, layer => layer.sections)
@@ -439,7 +441,7 @@ export class SystemPrompt extends Service {
     // Validate order against pre-restriction names while collecting visible schemas.
     const providers = [
       ...this.layers.global.toolProviders.values(),
-      ...(this.layers.peek(scope)?.toolProviders.values() ?? []),
+      ...this.layers.chainLayers(scope).flatMap(layer => [...layer.toolProviders.values()]),
     ]
     const collected: ToolSchema[] = []
     const knownNames = new Set<string>()