fix(tools,client): chain-mode resolution and a live preset label

modeFor read only the exact scope's layer, so a `code`-preset session
advertised the native catalog: the mode is declared on the preset's STANDING
scope, and the agent only parents to it. Nearest scope wins along the chain —
the mode decides what the model SEES, which is the class of fact the chain
inherits. Caught live (the model politely computed with bash and said
run_code was not in its list); the chain test pins it.

The client half of the label fix: the create echo and the session-added
frame's agentPreset now reach the session list (newest wins in the upsert —
every producer of the field reports the CURRENT composition), and a confirmed
blank-session switch publishes through the new ISessions.noteAgentPreset, so
the header label moves with the composition instead of waiting for a reload.

packages/client/test-runtime/src/sessions.ts

@@ -430,6 +430,14 @@ export class TestSessions implements ISessions {
     return Promise.resolve()
   }
 
+  /** Apply a confirmed preset switch into the fixture list, as production does. */
+  noteAgentPreset(sessionId: SessionId, agentPreset: string): void {
+    this.list.update((draft) => {
+      const summary = draft.byId[sessionId]
+      if (summary !== undefined) draft.byId[sessionId] = { ...summary, agentPreset }
+    })
+  }
+
   /** Clear the current selection (recorded; the production no-session flow). */
   clear(): void {
     this.calls.push({ method: 'clear', args: [] })