fix: deliver Claude teaching prose through an output tool

src/core/native-messages.ts

@@ -98,6 +98,55 @@ export function asAgentMessages(messages: readonly LhMessage[]): AgentMessage[]
 	return messages as unknown as AgentMessage[];
 }
 
+// ─── explicit learner-facing output ─────────────────────────────────────────
+
+export const SEND_TO_USER_TOOL = "send_to_user";
+
+/** Read the output tool's message without changing the native call. Also
+ * accepts partial arguments while streaming; malformed/empty inputs carry
+ * no prose. This projection is independent of the currently selected model. */
+export function sendToUserText(block: unknown): string | undefined {
+	if (!block || typeof block !== "object") return undefined;
+	const call = block as { type?: unknown; name?: unknown; arguments?: unknown };
+	if (call.type !== "toolCall" || call.name !== SEND_TO_USER_TOOL) return undefined;
+	if (!call.arguments || typeof call.arguments !== "object") return undefined;
+	const message = (call.arguments as { message?: unknown }).message;
+	return typeof message === "string" && message.trim() ? message : undefined;
+}
+
+/** Learner-facing prose, in content order. Only the explicit output tool
+ * joins ordinary text here — thinking and all other tool inputs stay out.
+ * Shared by coverage/extraction and Markdown; native history stays intact. */
+export function userFacingTextOf(content: unknown, failedOutputCalls?: ReadonlySet<string>): string[] {
+	if (typeof content === "string") return content.trim() ? [content] : [];
+	if (!Array.isArray(content)) return [];
+	const out: string[] = [];
+	for (const block of content) {
+		if (!block || typeof block !== "object") continue;
+		if (block.type === "toolCall" && failedOutputCalls?.has(block.id)) continue;
+		const text = block.type === "text" ? block.text : sendToUserText(block);
+		if (typeof text === "string" && text.trim()) out.push(text);
+	}
+	return out;
+}
+
+/** Failed output calls remain retry context, not delivered teaching.
+ * Truncated calls can have valid-looking partial arguments. Failed/aborted
+ * generations do not execute their calls at all, so may have no result. */
+export function failedOutputCallIds(messages: readonly LhMessage[]): ReadonlySet<string> {
+	const failed = new Set<string>();
+	for (const message of messages) {
+		if (message.role === "toolResult" && message.toolName === SEND_TO_USER_TOOL && message.isError) {
+			failed.add(message.toolCallId);
+		} else if (message.role === "assistant" && (message.stopReason === "error" || message.stopReason === "aborted")) {
+			for (const block of message.content) {
+				if (block.type === "toolCall" && block.name === SEND_TO_USER_TOOL) failed.add(block.id);
+			}
+		}
+	}
+	return failed;
+}
+
 // ─── id minting ──────────────────────────────────────────────────────────────
 
 /** The app-wide message id shape (same as the UIMessage-era mint). */