fix(oracle-r4-caveats): wizard rewires presentation edges, chat QA proves, Pi leak

Three Oracle round-7 caveats:

1. Wizard view replacement now deletes old presentation edges before
   creating new field-based ones. Previously handleDataSave added new
   edges additively, so runtime could still hit the generic Notes view.
2. Chat sidebar QA assertion was vacuously true (previewRows >= 0 &&
   timeBadges >= 0). Now: when threads exist, preview count must equal
   thread count AND at least one relative timestamp must render.
3. Agent composer placeholder rebranded from 'Ask Pi...' to 'Ask the
   assistant...'. Closes residual Pi/LLM framing leak.

src/scenes/Agent.tsx

@@ -110,7 +110,7 @@ export default function Agent() {
                 submit(draft);
               }
             }}
-            placeholder="Ask Pi to navigate, start a process, or message a teammate. Enter to send."
+            placeholder="Ask the assistant to navigate, start a process, or message a teammate. Enter to send."
             rows={2}
             disabled={working}
           />