Diffs: add viewer payload validation and presentation defaults

docs/tools/diffs.md

@@ -92,7 +92,10 @@ Set plugin-wide defaults in `~/.openclaw/openclaw.json`:
           defaults: {
             fontFamily: "Fira Code",
             fontSize: 15,
+            lineSpacing: 1.6,
             layout: "unified",
+            showLineNumbers: true,
+            diffIndicators: "bars",
             wordWrap: true,
             background: true,
             theme: "dark",
@@ -109,7 +112,10 @@ Supported defaults:
 
 - `fontFamily`
 - `fontSize`
+- `lineSpacing`
 - `layout`
+- `showLineNumbers`
+- `diffIndicators`
 - `wordWrap`
 - `background`
 - `theme`