fix(agents): wrap each reasoning line separately in italics for Telegram

Fixes #2060

Reasoning blocks display raw underscores instead of italic text on Telegram
because underscore markdown (_..._) cannot span multiple lines.

This change wraps each non-empty line separately in underscores, which
Telegram correctly renders as italic text.

Example output:
  Before: Reasoning:\n_line one\nline two\nline three_
  After:  Reasoning:\n_line one_\n_line two_\n_line three_

Added 7 tests to verify the new behavior.

src/agents/pi-embedded-utils.test.ts

@@ -1,6 +1,6 @@
 import type { AssistantMessage } from "@mariozechner/pi-ai";
 import { describe, expect, it } from "vitest";
-import { extractAssistantText } from "./pi-embedded-utils.js";
+import { extractAssistantText, formatReasoningMessage } from "./pi-embedded-utils.js";
 
 describe("extractAssistantText", () => {
   it("strips Minimax tool invocation XML from text", () => {
@@ -508,3 +508,41 @@ File contents here`,
     expect(result).toBe("StartMiddleEnd");
   });
 });
+
+describe("formatReasoningMessage", () => {
+  it("returns empty string for empty input", () => {
+    expect(formatReasoningMessage("")).toBe("");
+  });
+
+  it("returns empty string for whitespace-only input", () => {
+    expect(formatReasoningMessage("   \n  \t  ")).toBe("");
+  });
+
+  it("wraps single line in italics", () => {
+    expect(formatReasoningMessage("Single line of reasoning")).toBe(
+      "Reasoning:\n_Single line of reasoning_",
+    );
+  });
+
+  it("wraps each line separately for multiline text (Telegram fix)", () => {
+    expect(formatReasoningMessage("Line one\nLine two\nLine three")).toBe(
+      "Reasoning:\n_Line one_\n_Line two_\n_Line three_",
+    );
+  });
+
+  it("preserves empty lines between reasoning text", () => {
+    expect(formatReasoningMessage("First block\n\nSecond block")).toBe(
+      "Reasoning:\n_First block_\n\n_Second block_",
+    );
+  });
+
+  it("handles mixed empty and non-empty lines", () => {
+    expect(formatReasoningMessage("A\n\nB\nC")).toBe("Reasoning:\n_A_\n\n_B_\n_C_");
+  });
+
+  it("trims leading/trailing whitespace", () => {
+    expect(formatReasoningMessage("  \n  Reasoning here  \n  ")).toBe(
+      "Reasoning:\n_Reasoning here_",
+    );
+  });
+});

src/agents/pi-embedded-utils.ts

@@ -211,7 +211,13 @@ export function formatReasoningMessage(text: string): string {
   if (!trimmed) return "";
   // Show reasoning in italics (cursive) for markdown-friendly surfaces (Discord, etc.).
   // Keep the plain "Reasoning:" prefix so existing parsing/detection keeps working.
-  return `Reasoning:\n_${trimmed}_`;
+  // Note: Underscore markdown cannot span multiple lines on Telegram, so we wrap
+  // each non-empty line separately.
+  const italicLines = trimmed
+    .split("\n")
+    .map((line) => (line ? `_${line}_` : line))
+    .join("\n");
+  return `Reasoning:\n${italicLines}`;
 }
 
 type ThinkTaggedSplitBlock =