fix(consult): drop traces, budget checkpoints, silent summary inherit

BUG-729: dropped history rounds leave an omission marker in the model-visible summary slot.
BUG-730: checkpoint threshold is 0.4 of historyBudgetChars (128k still 16,000).
BUG-731: session_full new chat copies owned context_summary on the server; clients send only continued_from_session_id.

frontend/tests/chat-session-url.test.ts

@@ -173,7 +173,7 @@ test("popstate to a missing session query reuses selectSession side effects for
 });
 
 test("creating and leaving a session keep the address bar in sync", () => {
-  const startNewChat = sourceBetween(page, "async function startNewChat()", "function selectSession(");
+  const startNewChat = sourceBetween(page, "async function startNewChat(", "function selectSession(");
   assert.match(startNewChat, /writeSessionUrl\(nextSession\.id, "push"\)/);
   assert.match(startNewChat, /window\.history\.replaceState\(null, "", previousHref\)/);