fix(telegram): stop auto-linking @mentions to t.me profile URLs

LLM @mentions are not necessarily Telegram usernames. Auto-wrapping
them in <a href="https://t.me/..."> caused unwanted profile cards.
Keep placeholder protection for italic conversion, restore as plain text.

internal/channels/telegram/format.go

@@ -105,11 +105,11 @@ func markdownToTelegramHTML(text string) string {
 	// Strikethrough
 	text = regexp.MustCompile(`~~(.+?)~~`).ReplaceAllString(text, "<s>$1</s>")
 
-	// Restore @mentions as clickable Telegram links
+	// Restore @mentions as plain text (protected from italic conversion above).
+	// Do NOT wrap in <a href="https://t.me/..."> — LLM @mentions are not
+	// necessarily Telegram usernames and auto-linking shows unwanted profile cards.
 	for i, mention := range mentionPlaceholders {
-		username := strings.TrimPrefix(mention, "@")
-		linked := fmt.Sprintf(`<a href="https://t.me/%s">%s</a>`, username, mention)
-		text = strings.ReplaceAll(text, fmt.Sprintf("\x00MN%d\x00", i), linked)
+		text = strings.ReplaceAll(text, fmt.Sprintf("\x00MN%d\x00", i), mention)
 	}
 
 	// List items

internal/channels/telegram/format_test.go

@@ -62,6 +62,58 @@ func TestRenderTableAsCode_Vietnamese(t *testing.T) {
 	}
 }
 
+func TestMarkdownToTelegramHTML_Mentions(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string // substring that MUST appear
+		deny  string // substring that must NOT appear
+	}{
+		{
+			name:  "mention stays plain text",
+			input: "Hello @viettran how are you?",
+			want:  "@viettran",
+			deny:  `href="https://t.me/`,
+		},
+		{
+			name:  "mention not wrapped in link tag",
+			input: "cc @john please review",
+			want:  "@john",
+			deny:  "<a ",
+		},
+		{
+			name:  "email not treated as mention",
+			input: "send to user@domain.com",
+			want:  "user@domain.com",
+			deny:  `href=`,
+		},
+		{
+			name:  "mention survives italic conversion",
+			input: "_italic_ and @bot_name end",
+			want:  "@bot_name",
+			deny:  `<a `,
+		},
+		{
+			name:  "multiple mentions plain text",
+			input: "@alice and @bob discussed",
+			want:  "@alice",
+			deny:  "t.me",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := markdownToTelegramHTML(tt.input)
+			if !strings.Contains(got, tt.want) {
+				t.Errorf("expected %q in output, got: %s", tt.want, got)
+			}
+			if tt.deny != "" && strings.Contains(got, tt.deny) {
+				t.Errorf("unexpected %q in output, got: %s", tt.deny, got)
+			}
+		})
+	}
+}
+
 func TestChunkHTML(t *testing.T) {
 	tests := []struct {
 		name   string