all blocks use markdown rendering by default

- Tool blocks: expanded via sub-blocks (same as assistant) so embedded
  diffs and code fences render properly with coloring
- Call blocks: always JSON, use formattedContent (pretty-print + highlight)
- Prose sub-blocks use Text.MarkdownText for native backtick/bold/link support

gui/main.qml

@@ -601,18 +601,22 @@ ApplicationWindow {
         id: callDelegate
         ColumnLayout {
             spacing: 2
+
+            property bool expanded: false
+
             Label {
-                text: (callText.visible ? "\u25BC" : "\u25B6") + " call: " + (block.blockName || "")
+                text: (parent.expanded ? "\u25BC" : "\u25B6") + " call: " + (block.blockName || "")
                 font.pixelSize: Qt.application.font.pixelSize * 0.8
                 opacity: 0.5
                 MouseArea {
                     anchors.fill: parent
                     cursorShape: Qt.PointingHandCursor
-                    onClicked: callText.visible = !callText.visible
+                    onClicked: parent.parent.expanded = !parent.parent.expanded
                 }
             }
+
+            // Call args are always JSON - use pretty-printed highlighted HTML
             Text {
-                id: callText
                 Layout.fillWidth: true
                 text: block.formattedContent || ""
                 textFormat: Text.RichText
@@ -620,7 +624,7 @@ ApplicationWindow {
                 font.family: "monospace"
                 font.pixelSize: Qt.application.font.pixelSize * 0.85
                 opacity: 0.7
-                visible: false
+                visible: parent.expanded
                 color: palette.text
             }
         }