feat(x): graceful wrap-up when a chat turn hits its model-call limit

Interactive turns no longer die with a raw 'Model call limit of N
reached' error. The final budgeted model call is composed as a wrap-up:
the durable request records wrapUp: true, and the composer strips tools
and appends a notice telling the model to answer with what it has,
state what's unfinished, and mention the limit is configurable in
Settings -> Advanced. The turn then completes normally with that answer.

Headless turns (and a wrap-up call that still emits tool calls) keep
the hard model-call-limit failure so automation retains the
machine-readable outcome; sub-agents already surface partial results.
If a hard limit failure still reaches the chat UI, the error bubble now
explains the limit and points at the setting instead of the raw error.

apps/x/packages/shared/src/turns.ts

@@ -233,8 +233,26 @@ export const ModelRequest = z.object({
     contextRef: TurnContextRef.optional(),
     messages: z.array(ModelRequestMessageRef),
     parameters: z.record(z.string(), z.json()),
+    // Final budgeted call of an interactive turn: the composer strips tools
+    // and appends wrapUpNotice() so the model ends with a real answer
+    // instead of the turn failing with model-call-limit (issue #768).
+    wrapUp: z.literal(true).optional(),
 });
 
+// The message appended to a wrap-up call's request. A pure function of the
+// turn's durable config, so replaying the log reproduces the wire bytes.
+export function wrapUpNotice(
+    maxModelCalls: number,
+): z.infer<typeof ConversationMessage> {
+    return {
+        role: "user",
+        content:
+            `[system notice] You are on the final model call this turn's budget allows (model-call limit: ${maxModelCalls}); tools are no longer available. ` +
+            "Give your best final answer using only the work above: present what you completed, state clearly what remains unfinished, " +
+            "and let the user know the turn was cut short by the model-call limit, which they can raise in Settings → Advanced.",
+    };
+}
+
 export const ModelCallRequested = z.object({
     type: z.literal("model_call_requested"),
     turnId: z.string(),