Merge branch 'stack/agent-profiles-1-seam' into stack/agent-profiles-3-wire

Rewires this layer onto the standing-mount model:

- serviceForAgent roots its search at the agent's standing mount (parent
  scope key → live mount fiber) — the composition no longer lives under the
  agent's own fiber, and two agents on one preset now address ONE instance,
  which the sharing test asserts instead of distinctness.
- viewFor/historyPage take a registry view SCOPE. A live agent is that scope;
  a cold read uses the recorded preset's standing key via standingKeyFor —
  composing plugins but starting no agent, session, or turn. A header without
  a preset (a pre-roster log) renders through the DEFAULT preset's standing
  layer; an unusable preset degrades the read to generic cards, never fails
  it. This turns produced-files and chat-scroll green structurally, with the
  token counts untouched (no resume, so the projections fold stays detached).
- The detached projections baseline now includes every standing unit's key at
  its empty fold (todos: null): the standing mount registers units
  deterministically, which is what makes the client's "omitted key =
  capability absence → clear" rule safe again. seeded-history's contract test
  asserts the new shape.
- The standard preset's realm preamble no longer claims a shared label pools
  instances — provide() throws on the second registration under one realm
  symbol; labels join REALMS.

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

@@ -440,9 +440,11 @@ 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)
@@ -450,7 +452,7 @@ 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>()